A leak is information about a game that has been released prematurely.
Super Smash Bros. Wii U/3DS
There two leaks, the Gematsu Leak and the Bowser Jr. Leak (Bowser Jr., Shulk, Ganondorf, Duck Hunt Dog, etc.). The Gematsu Leak is believed to be fake after the problem of Chrom being part of Robin's Final Smash. The other part of this disaproval is the reveal of Rosalina. The latest leak says Bowser Jr., Duck Hunt Dog, Shulk, Ganondorf, Shulk (revealed), Ness, Mr. Game and Watch, R.O.B., and Wario are confirmed. There are leaks floating around the internet, even actual footage! These can be found on NEOGAF.