The Mistress

Basically the step up from Your Cheating Heart, but not up there with a concubine. It's a girlfriend for an already married man. Sometimes it's public, but just as often as not.

The reasons can vary. Often the husband just doesn't care enough for his wife, especially if there was an Arranged Marriage. Sometimes it's actually expected in a culture (even if not actually said). Sometimes the wife can even drive the husband to this, but just as often she can be a kind woman, or even a Hot Consort.

Sometimes the mistress will be happy with her position, but sometimes she will want the husband to leave his wife and marry her. Sometimes the latter could even lead to trying to outright get rid of the wife. Needless to say, if the wife wasn't aware of her existence and finds out about her, The Mistress is very likely to be killed at her hands, if the husband isn't the one to die that is.

Crime Fiction stories often involve this, as it conveniently gives everyone involved a motive for murdering at least one of the triangle. The more mistresses, the more potential red herrings/victims.

Remember, this has to be a mistress, not just an affair. That's Your Cheating Heart.

Note that this is NOT the owner of a servant, even if the word "Mistress" is supposed to be the feminine version of the word "Master".

Compare The Unfair Sex, Oops, I Forgot I Was Married (if the mistress does not know about the other's family), Dark Mistress (who is not an evil version of this, despite the name).

Examples of The Mistress include:

Anime and Manga

This trope is referenced in Ranma ½. One of the things Ranma does to try and get Ukyo to dump him is to tell her he's going to go spend the night with his mistress. Made funnier when he enters Akane's room and tries to convince her to go along with it. She really doesn't take it well.
Rosario + Vampire makes use of it overtly; in the case of those seeking Tsukune's love only one has expressed real interest in marriage, with the others either outright saying they want to be something else to him, or simply showing no great interest either way. However more subtly when we finally discover the full picture of the Shuzen family, we learn that not only was Issa casually polygamous, but it is implied that among his partners, Moka's mother Akasha Bloodriver is Issa Shuzen's mistress rather then his wife. This despite also being a Shinso vampire and one of the Three Dark Lords, who control/watch over Japan.

Film

Lola's father has one in Run Lola Run.
Helen's husband Charles has one in Diary of a Mad Black Woman , and she even had two children with him(meanwhile Helen had two miscarriages).
The book in Down With Love causes even mistresses to not cooperate with men anymore.
Sam's mistress in Ruthless People, who tries to blackmail him because she's sleeping with another man.

Literature

Tom's relationship with Myrtle in The Great Gatsby.
In Katherine Kerr's Devvery series, Rhodry takes Jill as a mistress—he would have liked to marry her, but could not for political reasons. It's acknowledged that this is common among the nobility due to the proliferation of political marriages; and in many cases the jilted spouse doesn't particularly mind as long as you are tactful and discreet.
One interpretation of the poem "The Rival" by Sylvia Plath (in Ariel) is that the subject is an Expy of the mistress of Plath's husband.
Making Money: The bank's previous owner was the mistress of the former chairman, mentioning that as his mistress, she had the wife's approval because it got him out of her hair for a while. She was also good with numbers, which is why she remained in charge of the bank after his death. Before she passed on, she noted that standards have dropped and that now "the ability to spin upside down on a pole is considered sufficient".
The position is Deconstructed in Dune wherein Paul Atreides considers the woman who is technically his mistress as his true wife, and never consummates his official (and political) marriage with his actual wife.
Petra Cotes in One Hundred Years of Solitude, doubling as one of the three hookers with hearts of gold.
In Guilty Wives, Winnie is revealed to have been the mistress of the President of France for some time before the action of the novel begins.

Live-Action TV

Spoofed in Designing Women, when they get a mistress to stop seeing a married man, but the wife was angry, because she knew about the affair and was having her own.
In the first time in Quantum Leap where Sam leapt into a woman, he was there to save "his" roommate from committing suicide over an relationship with her boss - she thought her boss would leave his wife for her, but the wife made it clear to her that she tolerated his "indiscretions" but they would never break up (and she was a former mistress of the boss herself, having broken up his previous marriage).
An extremely rare Gender Flip version appears on The Office, during the plotline where Michael thinks his girlfriend Donna is cheating on him but it turns out that the "other man" is her husband: "I'M the mistress?"
Kaamelott: Arthur has up to four mistresses at any given time, going from one to the other as his needs warrant. Hilarity Ensues when a rumor spreads of his rebuking the queen, and each girl tries to convince him to marry her.
Midsomer Murders loves this trope along with general adultery.
The Sopranos: Almost every made guy has a "gumar" on top of their wives who they won't divorce for their Catholic beliefs. For the main character, it becomes a major plot device when one of his mistresses actually contacts his home pushing his wife to call it quits, albeit temporarily.
- "Comare." Literally "godmother" in Sicilian dialect, but slang for mistress.
Desperate Housewives wouldn't really be Desperate Housewives if it didn't have at least one mistress per season.
In the Firefly episode "Shindig," Atherton Wing wanted Inara for one of these. Mal objected, leading to a sword duel over her honor.

Theatre

Cyrano De Bergerac: De Guiche wants to bully heroine Roxane into this. She will have none of it.

Video Games

King's Mistress is one of the endings in some of the Princess Maker games.
- Not only that, the second game as a possible ending with the girl as the lover of a local landlord as well. Said landlord can potentially show up earlier and make the girl his lover if she has high Charisma with low Morals—it's not recommended to just let her be, though, as her stats and overall rep will go lower.
- In said second game the King of the Land also has a royal concubine, and your daughter can speak to her. She seems to be a pretty but deadly bored woman rather disenchanted with the life of the court, but unwilling to leave it due to the benefits it still gives her.
Fire Emblem Blazing Sword: King Desmond of Bern has a son (Zephiel) by the queen (Helenne), and a daughter (Guinevere) by his unnamed mistress. His illegitimate family lives in the palace, while his official one lives in an old out-of-the way manor. It's explained in the prequel Sword of Seals that said mistress was actually the King's old girlfriend, whom the King couldn't marry due to being forced in an Arranged Marriage with the Queen.. Needless to say, it does not end well.
In Dragon Age, if you make Alistair king and cannot/will not make yourself queen and are in a romance, and provided you "hardened" him earlier, you may become his mistress. Not just a mistress, though, you may be his adviser, as well.
In Tales of Symphonia, Zelo's father had a mistress, who was the mother of Zelo's half-sister Seles. Despite him being the Chosen, it's implied that it was something everyone knew, as Zelos and Seles not only knew about each other, but they got on rather well. The mistress wasn't happy with the arrangement though, knowing that Zelos would be The Chosen One instead of their daughter. Let's just say It Got Worse and leave it at that.

Real Life

There are a lot of historical examples, because European aristocratic marriages were almost always purely political—it was generally accepted that the husband and (though less tolerated) wife would take lovers to provide the emotional fulfillment they would not be getting from their marriage. It was usually considered bad form to be public about it, but not always—in some countries such as France The Mistress was even something of a semi-official position. (Among other things, this made it easier to keep tabs on any illegitimate children that might one day contest for the throne...)
The picture is of Madame de Pompadour, one of the mistresses of Louis XV, who was featured in Le Chevalier d'Eon. She also appeared in the Doctor Who episode "The Girl In The Fireplace".
- The Doctor even mentions that she and the queen were good friends.

The Doctor: France! It's another planet!

- Queen Marie wasn't happy with her husbands affairs at all, since she was a devout Catholic. She merely preferred the well-behaved and properly deferential Pompadour over her downright arrogant predecessors. Marie Therese, on the other hand, was openly disappointed when her husband exchanged Louise de la Valliere for de Montespan.
- Madame DuBarry was another mistress late in his life, who was an initial antagonist in Rose of Versailles.
Looking through the history of any European country, the amount of mistresses the royalty and nobility kept on hand is staggering. Special mention to Louis XIV of France, who was still sleeping around at 70, and Augustus II of Poland, who ended up with 350 children, of which one was legitimate.
King Henry VIII had a confirmed two mistresses, Elizabeth Blount and Mary Boleyn (yes, sister to Anne Boleyn). He may have had other mistresses and considering his personality that wouldn't be surprising but beyond Elizabeth and Mary it is purely speculation.
The most famous example of a queen openly keeping lovers is Catherine the Great. She also rewarded her lovers handsomely (one of them briefly became king of Poland). Notably she started before she had her husband overthrown (and possibly murdered) and kept it up afterward. (This is the likely point of the rumor that she died, um.. 'ahorse' -- it was a joke about her insatiable appetite.)
- As well as her penchant for picking her lovers from her horse guards.
A Swedish example: Frederik I's mistress Hedvig Taube who (like many others) acquired a large fortune from her liaison with the king.
King Charles II of England, Scotland, and Ireland was known as "the Merry Monarch" for his free-living lifestyle, and is famous for having twelve acknowledged illegitimate children by seven mistresses—with no less than five of the children being by Barbara Palmer, Countess of Castlemaine—and no children at all by his legitimate wife, Catherine of Braganza. Many of Charles' bastards have prominent descendants, with one of them (Diana Spencer) marrying the Prince of Wales (making Princess Di), and thus probably finally getting one of Charles' descendants in line for the throne.
William IV of the United Kingdom was also famous for a large number of illegitimate children (as, frankly, were many of the sons of George III, much to their devout and Happily Married father's disgust—at least when he was lucid enough to be disgusted). He had only two daughters by his wife, Adelaide of Saxe-Meiningen, but he had ten by his mistress, the actress Dorothea Jordan. Both of his legitimate daughters died, leaving the throne to his niece Victoria. And of course, William's surviving children had important descendants, the most famous of whom is current Prime Minister David Cameron.
Marion Davies, longtime girlfriend of publishing magnate William Randolph Hearst. Hearst remained married to his wife Millicent until his death, despite having been carrying on with Davies for the last thirty-four years of his life.