Internet slang: A unknown fact within a series (generally of a future event) that ruins the tension and mystery for newer fans.
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, Luke" >This statement is WHO+WHO and thus doesnt convey a spoiler as no information is being revieled.
"Vader is Luke's Father" >This is WHO+WHO+WHAT, and thus fits the "3 of 6 rule" and is a spoiler.