Spoiler

Spoilers were things one individual knew about another's personal future. River Song particularly favoured the term, and used it to warn the Tenth (TV: Silence in the Library) and Eleventh Doctor (TV: The Name of the Doctor) when they asked questions that she couldn't answer without revealing details about their futures. The Tenth Doctor first heard her use the term in the Library, (TV: Silence in the Library) but she first heard it from the Eleventh Doctor in Berlin. (TV: Let's Kill Hitler)

The Ninth Doctor used the word while chiding the Twelfth Doctor for revealing that he would move the Earth in his next incarnation. (COMIC: The Lost Dimension)

The Tenth Doctor also used the term to refer to reading books from Donna Noble's future; the Doctor taking a 51st century book away from Donna so as not to read ahead and "spoil all the surprises". Donna questioned if travelling in time with the Doctor was "one big spoiler". The Doctor responded that he tried to keep her away from major plot developments, which he also said he seemed to be "very bad" at. (TV: Silence in the Library)

The Eleventh Doctor used this to the Tenth Doctor when he asked him how he could forget after 400 years how many children there were on Gallifrey the day they remembered the War Doctor destroying the planet. (TV: The Day of the Doctor)

While the Twelfth Doctor and River Song viewed the Singing Towers on Darillium, the latter asked if the rumours she'd looked up were true that this would be their final night together. The Doctor deflected the question with this phrase, though River took it as a confirmation, which the Doctor denied a few moments later. (TV: The Husbands of River Song)

Confusing Captain Archibald Hamish Lethbridge-Stewart by referring to the conflict in which he was currently engaged as World War One, the Twelfth Doctor apologised for spoiling him. (TV: Twice Upon a Time)

After the Thirteenth Doctor chided a swarm of Weeping Angels for everything that they had taken away from her in the past, the Tenth Doctor, who had yet to experience said losses, enquired as to what she was referring to, she used the term. (COMIC: A Little Help from My Friends)

Of course, neither the Doctor nor River was the originator of the term. The Doctor had certainly heard and used it long before meeting River. While being tortured by a member of Faction Paradox in San Francisco, the Eighth Doctor strongly cautioned him against revealing any spoilers. (PROSE: Unnatural History) The Tenth Doctor also said the term to Donna shortly before River's archaeological expedition arrived on the Library. (TV: Silence in the Library)

Other companions besides River also knew the term. Fitz Kreiner once wrote a song called "Contains Spoilers" which he assured his audience did indeed contain spoilers. (PROSE: The Gallifrey Chronicles)

Alternate meanings
In the newspaper trade, spoilers were a marketing tactic. According to James Stevens, when one newspaper scored an exclusive in an early edition, competing newspapers would spoil their competitors by printing the substance of the scoop in an edition that came out mere hours later. (PROSE: Who Killed Kennedy)

In television, a spoiler was any information about an episode that had yet to be broadcast. KWJM3 broadcast an hour-long trailer for their upcoming fall season, something that was full of spoilers about upcoming content. (PROSE: Synthespians™)

Behind the scenes
Steven Moffat included the reference in Silence in the Library as a slight dig at fans who wanted to know details of forthcoming episodes of Doctor Who before they aired. (CON: Shadow Play)

Information from invalid sources
Big Finish were not allowed to discuss details of their audio story The Doomsday Contract, due to spoilers. (NOTVALID: Tom Baker stars in John Lloyd's lost Doctor Who adventure, The Doomsday Contract)

Spoiler ספוילר