Can someone point me to the definition of a "spoiler?"

[David]

In several places I have run across the term "spoiler" as though I should know what that is. But I don't. Who can define it for me?

I know what the spoiler is on a car, and on a plane, but not on a book. :-)

[Amanda]

From Free Dictionary

Spoiler:

5. A published piece of information that divulges a surprise, such as a plot twist in a movie.

[Banjomike]

In the trivia and quizzes areas it would include clues about who the murderer was, who dies, who wins at the end, who are the likely blackmailers, etc. Some spoilers in the past have included the punchlines from the last book of a 10 volume series, and the plot twists at the end of Agatha Christie novels. Anything like that gets deleted on sight.

Remember that trivia and quizzes are seen before someone makes a decision either to answer or NOT answer them. The question itself can be a major spoiler even if the answer is not attempted and not known.

[Robin]

It is just as it sounds: a part of the book which revealed to you before reading it yourself will "spoil" it for you. It does not necessarily have to be a big surprise or twist, just something you would rather read yourself than hearing about from someone else and thereby "ruining" at least part the reading experience for you. Some spoilers are bigger than others and some people are more strict in what they consider a spoiler. When in doubt, use the spoiler tag: <*spoiler> text here <*/spoiler> (remove asterisks). If you wouldn't want it revealed to you before reading/watching it for yourself then be considerate and don't blab about it publicly and ruin it for other people.

This can apply to movies or TV shows as well - like revealing who Luke's father is or the ending of Sixth Sense - those are major one. Even sports scores and results - if you're recording a game to watch later and then someone tells you the final score - it ruins or spoils it for you.