spoiler

Short definition from Wiktionary

Noun

1. 1
   One who spoils; a plunderer; a pillager; a robber; a despoiler.
2. 2
   One who corrupts, mars, or renders useless.
3. 3
   A document, review or comment that discloses the ending or some key surprise or twist in a story, or the internal rules controlling the behaviour of a video game, etc.
4. 4
   A device to reduce lift and increase drag.
5. 5
   A device to reduce lift and increase downforce.
6. 6
   A competitor, unable to win themselves, who spoils the chances of another’s victory.

Verb

1. 1
   To mark (a document or message) with a spoiler warning, to prevent readers from accidentally learning details they would prefer not to know.
2. 2
   To tell (a person) details of how a story ends etc.