Metaweb:Spoilers

From the Quicksilver Metaweb.

A spoiler is a comment which reveals important plot points or the solution to puzzles. It is considered courteous to warn readers at the start of an article when that article contains spoilers.

If, for example, one was discussing a murder mystery and wanted to make comments regarding the identity of the killer, it would be polite to include a spoiler warning so as not to diminish the enjoyment of the book by those who read the article before the book itself.