Generally, a spoiler is anything which reveals an ending or ruins a surprise. In the Dungeon Crawl sense, a spoiler is a file documenting the behaviour of the game in great detail. Hundreds of different spoilers exist, from tables of weapons and monsters, to FAQs, to comprehensive guides. Many are now out of date; be sure to check which version of Dungeon Crawl the spoiler was written for.
CrawlWiki articles, collectively, constitute a spoiler.