Here is a belief of mine that I think is pretty uncontroversial but which, it turns out, my friendship group contains some pretty heated disagreement on. A spoiler for some piece of fiction is any bit of information (which pertains to events depicted) for which being told it beforehand significantly affects your experience of the fiction.
(Don't read too much into the `significantly' - I am just friends with philosophers, so have to qualify to rule out irritating Cambridge-spoilers; I don't think the difference between `experiencing the fiction knowing X' versus `experiencing the fiction not knowing X' is significant in all cases, and if you're being real neither do you. Ok.)
I think this definition broadly matches popular usage and some popular attempts at definition -- for instance this. But apparently when one draws out its consequences it becomes pretty controversial pretty quickly. Some examples of said controversial consequences.
First, historical information can constitute a spoiler. Knowing that Ceasar gets stabbed, the Titanic sinks, and that a complex series of battles, parliamentary reversals, and marriages, results in a Lancastrian monarchy can all, in the right context, spoil works of fiction.
Second, we'll only know what all the spoilers are once we're dead. We never know what information we are gaining now could turn out, in future, to affect our experience of some piece of fiction. Everything you learn is potentially a spoiler for some future tale. Life in a democracy is full of risks, and this is one of them.
Third, not quite a consequence but close: one can fully permissibly spoil things, it is not the case that it is always bad to spoil a work of fiction. Maybe it is bad always and everywhere to deliberately spoil a work of fiction (even if this bad can be overrode by other goods one thereby attains), but certainly giving away information which in fact constitutes a spoiler is not in itself even a prima facie bad in a great many scenarios. It may even be a good thing to do sometimes.
Fourth, spoiling is quite an individualistic affair. It depends on the peculiar character of the individual how they experience a work of fiction, and how their information bears on this; it does not depend (except in a derivative sense) on the intentions of the author of said fiction, nor on the nature of the information conveyed. Nothing is intrinsically a spoiler, it all depends on how it interacts with the individual and their mode of experiencing fiction.
Ok, there we go. I think all this is quite obvious, but frequent disagreement compels me to write it out in an easy to access place for future reference. Now you know, and knowing is half the battle.
EDIT: Thanks to Kenny Easwaran and Eric Schwitzgebel for pointing out ammendments which I have incorporated into this definition. Keep them coming!