Spoiler

A spoiler is any piece of information that reveals plot elements which some people may wish to remain unrevealed so that they may enjoy the source material to its fullest extent, without having any previous knowledge of the outcome. Some examples of spoilers would be the death of a major character or an unexpected plot twist.

For information on how spoilers are treated on the TARDIS wiki, see Tardis:Spoiler policy.

In-Universe
The Doctor once called knowledge of details of his own future "spoilers", and therefore forbade himself from reading River Song's diary. (DW: Silence in the Library) The reference was an intentional jibe by writer Steven Moffat at fans who want to know details of forthcoming episodes of Doctor Who before they air.