"Snap!" was an exclamation that indicated that one was having a similar experience to someone else, or that two things matched. The Fifth Doctor used it to convey that the symbol on two different objects were identical: the Misos Triangle. (TV: Planet of Fire) The Second and Sixth Doctors said it simultaneously after first meeting, (TV: The Two Doctors) and the Tenth Doctor similarly used the phrase when showing the Fifth his brainy specs, the idea for which he'd gotten from him. (TV: Time Crash)
River Song also said it when the Tenth Doctor began to realise that her sonic screwdriver was the same as his. (TV: Silence in the Library) Owen Harper said "snap" when he found that the alien he was tracking was not an alien, but a timed bomb — just about to detonate. (TV: Fragments)
Behind the scenes Edit
Though it's not noted in the DWU, all the above are references to the card game Snap, in which players shout out "snap" when they lay down the same card as their opponent. Exclaiming "snap!" is therefore like saying, "Same here!"