The underlying idea or question your story explores - not what happens, but what it all means.
Theme is the central idea, question, or commentary that runs beneath the surface of your narrative. It's the difference between plot (what happens) and meaning (why it matters). A story's plot might be 'a man hunts a white whale,' but its themes include obsession, fate, the limits of human knowledge, and humanity's relationship with nature. Theme isn't a moral or a lesson - it's an exploration, and the best themes are complex enough that reasonable readers can disagree about what the story ultimately 'says.'
Theme is what separates a story that entertains from a story that resonates. It's the reason readers remember certain books years after finishing them, the reason a novel about wizards can make you think about prejudice, and the reason a detective story can explore the nature of truth. Without theme, you have a sequence of events. With it, you have something that speaks to what it means to be human.
The novel explores racial injustice, moral courage, and the loss of innocence - but it does so through the specific, concrete experience of Scout watching her father defend a Black man in 1930s Alabama. The theme lives in the story, never above it.
The novel's themes - the nature of truth, the mechanics of totalitarianism, the relationship between language and thought - are explored through Winston's personal story, not through essays disguised as fiction.
Guilt, redemption, and the possibility of atonement drive the narrative. The theme isn't stated - it's lived through Amir's decades-long journey from cowardice to courage.
If a character delivers a speech about your theme in the final chapter, you've written a sermon, not a story. Trust your narrative to carry the meaning. Show, don't tell - especially with theme.
'War' is a subject. 'A soldier returns from deployment' is a plot. 'War destroys the capacity for intimacy' is a theme. Theme is always an idea about the subject, not the subject itself.
Some writers discover their theme through the process of writing, not before it. If your story feels like it's in service to a predetermined message, it will read like propaganda. Let the story teach you what it's about.
A novel can explore several themes, but it usually works best with one primary theme and a few supporting ones. If every chapter is about a different big idea, nothing gets explored deeply enough.
Take something you're working on and write down what happens in one sentence (that's your plot). Then write down what it's really about in one sentence (that's your theme). If you can't articulate the theme, write three questions your story seems to be asking. Now look at your draft - does every major scene connect to at least one of those questions? Cut or revise any scene that doesn't.