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 and Eleventh Doctors when they asked questions that she couldn't answer without revealing details about their futures. (TV: Silence in the Library, The Name of the Doctor)

The Tenth Doctor first heard the term from River when they met in the Library, (TV: Silence in the Library) but River first heard it from the Eleventh Doctor in Hitler's Germany. (TV: Let's Kill Hitler)

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 when the Doctor destroyed it. (TV: The Day of the Doctor)

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 Little Brother of Faction Paradox in San Francisco, the Eighth Doctor strongly cautioned him against revealing any spoilers. (PROSE: Unnatural History)

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 as a slight dig at fans who wanted to know details of forthcoming episodes of Doctor Who before they aired. (CON: Shadow Play)

ספוילר Spoiler