Relationship Reveal

The point within a series where an existing couple goes from merely being implied (or Subtext) to being firmly entrenched in canon proving that they've actually been together for some time.

Shouldn't be confused with Relationship Upgrade, where the relationship is beginning just as the audience becomes aware of it; this is when the relationship started before the audience knew, but was hidden from the audience. A similar but different trope for a homosexual couple getting outed outside of a series run by the author is Word of Gay.

WARNING! There are unmarked Spoilers ahead. Beware.

Examples of Relationship Reveal include:

Anime and Manga

Naruto: It had long been hinted that Kurenai and Asuma were a couple and finally confirmed when Kurenai was revealed to be pregnant.
In Stray Little Devil Aregna and Lizyerra are hinted at then revealed to have a pre-existing relationship in volume five of the manga.
Sonsaku and Shuuyu, the ruler and strategist from the Son kingdom in Koihime Musou show a lot of unspoken intimacy towards each other which hint at them being lovers. It's in season 3: episode 7, that the two characters are explicitly shown to be long term lovers.
Sherry and Ren had some Ship Tease the first time they met in Fairy Tail, after seven years later are now engaged.

Comic Books

Wiccan (Billy Kaplan) and Hulkling (Teddy Altman) in the Young Avengers. Fans suspected that they were a couple from the first issue, but it wasn't made canon until the end of the first arc. It was originally going to be a bigger reveal, but so many people caught on that the writer didn't bother. Kate simply mentions that Billy, then going by "Asgardian", needs to change his codename to something that "won't become a national joke when the press find out about you and Teddy".
In an issue of X Factor, long-time Heterosexual Life Partners Rictor and Shatterstar put an end to a decade's worth of innuendo and editorial wank with an in-panel kiss. Shatterstar's co-creator Rob Liefeld was not pleased and declared his intention to undo it as soon as he can. The chances of him actually getting his hands on the characters by this point are fairly remote.
X-Men: Although Destiny is long gone, whenever she's mentioned, there's no ambivalence to the fact that she and Mystique were indeed lovers. It could be argued that this started as early as the Shadow King using an obsolete word for "lover" back in 1989, but by the 2001 Retool, it was done.

Fan Works

In the Katawa Shoujo fic From Shizune's Perspective, this happens when Shizune finds Hisao and Misha having sex.

Literature

In the Dragon Age tie-in novel The Calling, Julien and Nicolas are a pair of Grey Warden warriors with an intense friendship. They spend most of their time together, one is never mentioned without the other, etc. When Julien dies, Nicolas flies into a rage and is distraught. He picks himself up and keeps going, but when a Lotus Eater Machine brings Julien back to him, he opts to stay in the dream rather than return to reality.
In the Honor Harrington novel Honor Among Enemies, Citizen Admiral Javier Giscard and his assigned Citizen Commissioner, Eloise Prichart, are established to have a very antagonistic working relationship, albeit not without considerable mutual respect. One scene abruptly begins with the two of them lying in bed together sharing a bottle of wine, idly debating whether or not their crew has figured out that they're secretly sleeping together yet or not.

Live-Action TV

In the fourth season of Buffy the Vampire Slayer, Willow and Tara were only implied to be a couple for a fair while before it was finally revealed that they were actually a couple.
Claire Kincaid and Jack McCoy on Law and Order
Grissom and Sara on CSI
Beecher and Keller of Oz - what started out as merely slashy turned canon and stayed that way.
A minor example happens in Angel's penultimate episode, with Spike's line: "Me and Angel have never been intimate. Except that once."
Mulder and Scully get this in the last episode of season 8, where they kiss while holding their son. (Amazingly, some fans still tried to insist that was a friendly kiss. With the release of the 2008 movie, where they're sleeping in bed together and saying things like "that's why I fell in love with you," the last holdouts seem to have conceded.)
- However, exactly when their Relationship Upgrade was is anyone's guess, mostly due to the angst of season 8. It's a leading theory in the fandom that they started a relationship after season 7's "all things" (in which they finally sleep together) and simply hid it.
In Bones, Bones spends the night at Booth's house, but the audience doesn't find out if they slept together in the figurative sense or just the literal one until the season finale, when Bones reveals that she's pregnant with Booth's baby.
Glee has Santana and Brittany.
In Farscape, D'Argo and Chiana were shown to have growing feelings for each other and are implied to become intimate at the end of "Out of Their Minds", but it isn't until Crichton walks in on them having sex in "Look at the Princess Part 1" that it's confirmed.

Web Comics

Used in Flipside regarding Maytag and Bernadette's relationship.
Occurs in Drowtales with regard to Mel'arnach and Snadhya'rune, who had been implied before to be a couple and were later confirmed. Mel presents them as Star-Crossed Lovers because of their rival families, but knowing Snadhya (who killed her own mother) it may be more complicated than that...
Theo and Sleipnir from Girl Genius are almost always together, but it was unclear whether they were an item or Like Brother and Sister...until this strip.
- The Novelization of the first three volumes makes an early joke about their relationship, hinting that this might be a very minor Retcon.

Western Animation

Though they're technically different series, Ren/Stimpy became canon in Adult Party Cartoon.