What constitutes a spoiler is often debatable but a good rule for determining one is the "3 of 6 rule". The "six" are Who, What, Where, When, Why, and How. If a comment has atleast three of these six then it can generally be called a spoiler.
One of the only exceptions to the "3 of 6 rule" is "X dies" or "X is dead" as this is still considered a spoiler because the When or How is generally considered irrelevent when it comes to deaths.
"Vader is Luke's Father" >This is WHO+WHO+WHAT, and thus fits the "3 of 6 rule" and is a spoiler.
2. noun, A piece of information likely to ruin dramatic tension in any piece of entertainment if discovered too early. Usually any plot development revealed more than a third of the way through the story. She's a man, he's a ghost, it's his sledge, he's a clone, etc.
3. verb, To ruin any piece of entertainment by revealing such information.
4. verb, To protect others from entertainment being ruined by obscuring such information from them.
2. I would tell you who Kaizer Soze is, but it's a massive spoiler.
3. Please don't spoiler Bioshock Infinite for me, I can't play it until after my exams.
4. I wish you'd spoilered that Breaking Bad post, I'm only on season two.