Subtext (sometimes called undertone) is content of a work which is not announced explicitly by the characters (or author) but is implicit or becomes something understood by the observer of the work as the production unfolds. It literally means "under the text", just as if it were ideas buried under the text or dialogue of a work.
Subtext is content not included in the spoken dialogue or written word. A character's motivation may be revenge, anger, competitiveness, pride, or lust, but they're not always going to come right out and say it. Much of what the characters truly think is left implicit for the audience to understand. Creators often make use of metaphor, innuendo, and motifs to convey subtext.
Subtext is also a frequently used method of subtly inserting social or political commentary into fiction, often as a way to Get Crap Past the Radar. Sometimes these are Parental Bonus subtexts, meant to be picked up by a crowd that will understand them.
Use of subtext is generally considered to be good writing, and is a major part of Show, Don't Tell. Mind Screw works rely on subtext to convey a lot of their meaning, as it allows complexity to be layered on a work.
Subtext is present in literally every work of fiction and every human interaction. Only rarely does anyone explain the entire reasons behind their actions—and even if they do a lot of social context will still be implied.
- Here's a lampshaded example from Woody Allen's Annie Hall. The subtext was actually displayed on-screen with subtitles, making the awkward subtext of a typical Meet Cute part of the text.
- HER: Hi!
Oh, God, nobody cool says Hi.
- HIM: Hello!
I'd love to take her out. I hope my deodorant's working.
- HER: Nice view.
He's talking to me! And he'll hate my silly dress.
- HIM: (Taking a furtive step toward her) Just look at the clouds over there.
I've got to find out who she is.
- HER: I'm Leslie ...
What a stupid name. He'll hate it and hate me.
- HIM: Neat dress.
I just love her name.
- HER: Hi!
- Xena: Warrior Princess was famous for having a Lesbian Subtext involving romantic attraction between Xena, the protagonist, and Gabrielle, the sidekick. But that's why not why this is listed here. In Xena fandom circles, the word "subtext" was so strongly associated that it is used to refer to only that particular pairing.