Sexy Backless Outfit

The back is what's important!

—Director Mori Toshimichi on Noel Vermillion's design.

Backless outfits used for Fan Service. Ever since the 1920s and much of the 1930s popularized the backless dress or top, whether bias-cut, halter, spaghetti-strapped or strapless, it has been a nice way of showing a little skin (fair, olive, dark or tanned) without getting too raunchy; a deeply cut dress can show the entire back and maybe a bit more, and makes it clear that the lady is not wearing a bra. At the same time, it can be more elegant and mature than Bare Your Midriff or some of the other Fanservice Costumes. Or not.

The "elegant evening dress" version may come with Opera Gloves. Whether elegant or not, it may have slits to demonstrate that She's Got Legs.

Note that outfits that never have a back in the first place don't count (e.g., Chainmail Bikini).

Compare with Toplessness From the Back, which reveals as much skin to the viewer.

No real life examples, please; we'd be here all day.

Examples of Sexy Backless Outfit include:

Anime and Manga

Umineko no Naku Koro ni: Beatrice wears a full-skirted Victorian gown which shows a quite period-inappropriate amount of back.
Bleach: Yoruichi and Soifon both have special uniforms that leave their shoulder blades exposed in order to use Shunko, a special technique that concentrates energy on the arms and back, without ending up completely naked.
Naruto: We have yet to see it in-series, but one of the databooks shows that under her Black Cloak Konan wears a backless dress.
Karakuridouji Ultimo: Quite a few of the doji, especially Ultimo and Jealous, have backless tops. Might be Fan Disservice since it's a Shonen manga and they're male, but then again...
Rebuild of Evangelion: Asuka has a variant: the test-type plugsuit's torso material is incredibly sheer, and at one point she turns to show that it perfectly defines her back and the top of her rear. Yeah, Vapor Wear while your entire body is clothed is pretty impressive.
Birdy the Mighty: Birdy's leotard-like outfit, especially in the Decode remake, in which her outfit is not only completely backless, but neither the front part, nor what back there is, is held on by anything. Presumably there is glue involved. Or it's Body Paint.
Homura's devil outfit in Puella Magi Madoka Magica the Movie: Rebellion. While in general her outfit is stripperific, the no back is practical considering the wings (which themselves look impractical). Although the fact she is biologically likely to be 13-14 probably (and hopefully) undermines any fan service appeal. Having gone through many time lines and the amount of struggle she was put through might give a case she is mentally older than her biological age would suggest.

Film

Who Framed Roger Rabbit?: Jessica Rabbit wears a slinky strapless, backless, sequined red evening gown slit up the thigh to here. She's a Hello, Nurse! character (a Shout-Out to Tex Avery's designs), and very much played for sexy.
Iron Man: Pepper Potts buys herself a backless dress "from Tony" for her birthday. She wears it to a gala event, where Tony shows up and Hilarity Ensues when he asks her for a dance. She's rather nervous about it, and this is only exacerbated by the fact that she forgot to wear deodorant and is dancing with her Casanova boss in front of all the people she works with.
Catwoman: Halle Berry's corsetlike second costume was specifically cut so we could see her back move.
The Tall Blond Man With One Black Shoe (1973): A Femme Fatale spy meets her intended conquest at her door in a black dress that conceals everything from her neck to her wrists to her feet. Then she turns around, and her back is completely bare, down to butt-cleavage.
- Likewise done in the 1985 American remake, The Man With One Red Shoe.
Dinner At Eight (1933): Gold Digger Kitty Packard (Jean Harlow) says, during a party conversation about sunbathing: "You know, my skin's terribly delicate and I don't dare expose it." Then she turns around, revealing her backless dress.
Attack of the Clones: The dress Padmé wears at the lake house combines a halter top, backless to the waist, with Detached Sleeves and filmy drapery for an elegantly sexy look.

Literature

Kushiel's Chosen: Phedre's costume for the Longest Night masque is deliberately cut so the entirety of her marque (full-length back tattoo) is visible. In her culture, marques are sported by courtesans who have completed their apprenticeships. A fully bared marque means its wearer is seeking business offers. Her back (and a touch more) appears prominently on the cover.

Live-Action TV

The West Wing: Ainsley Hayes in one episode; this being an Aaron Sorkin show when Sam commented on how attractive she looked, that led to a discussion of feminism and sexuality in the workplace.

Sam Seaborn: Whoa, I didn't even see that thing from the back.

Tabletop Games

In the 3.5 of the Eberron Campaign Setting, the example character for the Dragonmarked Heir prestige class was depicted with such a dress, though for her; it had the added benefit of showing off her Dragonmark of Storm.
A couple of the Iconics for Pathfinder have similar outfits, though artwork showing them from behind doesn't happen too often. The coverart for Raiders of the Fever Sea (the second part of the Skull & Shackles Adventure Path), however, plays this trope straight.

Theatre

Murder Among Friends: Angela's red dress must be backless or nearly so, to inspire her husband's line "Lovely dress, but don't you have it on backwards?"

Video Games

Castlevania: Order of Ecclesia: Shanoa wears a backless halter-top dress with a deep slit up the side, with a Breast Plate over it which is designed along the lines of a corset. The backlessness is necessary because she absorbs Glyphs through a large and intricate tattoo on her back.
Samurai Shodown: Shiki wears a backless tight dress and her back sports a massive snake tattoo that... does nothing.
Neverwinter Nights: Aribeth's fantasy girly armor is rather open down the back. That suit of armor's + 7 AC bonus (before magical enhancements) is completely unjustified, as it exposes at least as much as it protects. It only covers the "important" areas.
Fatal Fury and the The King of Fighters: Mai Shiranui had one intro where she bares her back and more while wearing a traditional Japanese kimono. In KOFXIII, she finally gets to demonstrate this trope by having a barely-there backless outfit with Absolute Cleavage in the front and nothing but a Thong of Shielding and a long sash in the back.
Bayonetta: This game takes Third-Person Seductress to whole new levels; the eponymous character wears a variety of these, all made out of her hair.
Primal: Third-Person Seductress Jen features a midriff-baring backless red halter top which shows off her Power Tattoo. It's prominently featured on the cover.
Tales of Hearts: Kohak Hearts has a backless leotard which covers her to the neck in front, but, combined with the little leg-frills that suggest a microskirt, gives her a very flirty look from the back.
Lost Odyssey: Female pirate Seth Balmore's outfit seems to lack any sort of back above the waist, no doubt giving Jansen in the back row an eyefull.
BlazBlue: Noel's standard outfit in BlazBlue is backless with a halter top, Detached Sleeves and a tiny skirt. The top also opens down the front for an alternate look with bonus extensive cleavage.
Kingdom Hearts: Birth By Sleep: Aqua; it got to the point that Tetsuya Nomura actually had to censor the outfit's design. Her back used to go all the way down to below her waist.
Sonic the Hedgehog: Rouge boasts this feature on both incarnations of her Spy Catsuit, though it makes sense, since she has wings.
Final Fantasy X: In the original, Yuna's regular outfit is a halter-top, backless variant on traditional Kimino. Her Gunner outfit in Final Fantasy X-2 is much skimpier, backless, sleeveless, and has Absolute Cleavage to boot.
Suikoden V: The Prince's main outfit involves backless, sleeveless armor and long gloves. It probably doesn't help that he's a pretty Bishonen who takes after his Hot Mom.
Tekken: Anna Williams wears a strapless, backless cheongsam with a Cleavage Window and a deep slit up the side. In case you couldn't identify her as the resident Ms. Fanservice.
Skies of Arcadia: Aika has a perky backless minidress with the skirt styled in panels slit to the waist (and some very small Modesty Shorts). Fina has an elegant long, backless halter-top dress with a Cleavage Window.
God of War: Chains Of Olympus: Persephone has a black halter-top number that goes down the back well past the waist. It incorporates a Thong of Shielding to keep within R-rated territory. Barely.
Resident Evil 5: Excella has a backless halter-top evening dress with Absolute Cleavage and Sideboob that must require lots of tape to keep in place.
Sengoku Basara: Magoichi wears a backless corset which defies all the laws of physics by staying on her.
Ibara (and sequel): Shasta Rose's outfit is basically a backless leotard with a ruffled collar, sash, and just enough of a skirt to pose for Panty Shots.
Street Fighter: All of the Dolls and the Shadoloo version of Cammy (a.k.a. Killer Bee) wear an uniform with a nice gap on the back, making the back, aside from the face, the only portion of visible skin in an otherwhise more straight Spy Catsuit (Leotard of Power, in Shadoloo Cammy's case).

Visual Novels

Phoenix Wright: Ace Attorney:
- Adrian Andrews has a backless halter-topped blouse. Her glasses and notebook give her a Sexy Secretary vibe, although she's actually an "entertainment manager".
- Shih-na of Ace Attorney Investigations wears a deeply-slit backless evening gown she wears in her role of Femme Fatale mole.

Webcomics

Sabine of the Linear Guild wears a backless, strapless red leather outfit almost all the time. Partly justified in that she has demonic bat wings part of the time; observe Sabine in human guise here and Sabine in her true demonic form here.

Western Animation

The Simpsons: In the "Large Marge" episode (which involved her getting Gag Boobs and becoming a Booth Babe), Marge Simpson wears a sexy backless red dress that emphasizes her new "attributes" for a public appearance.
Betty Boop was perhaps the first animated character to have this: a skimpy red strapless, backless dress that had to be lengthened to meet the Hays Code.
Red, of Tex Avery's Red Hot Riding Hood and other shots; a Hello, Nurse! character in a skimpy red strapless, backless Burlesque outfit.
Tripping the Rift: Resident Ms. Fanservice Six always wears outfits that shows off her back (and front, as well as most of her body).
Ahsoka Tano's redesigned outfit from Star Wars: The Clone Wars.