An official knockdown is ruled in boxing when a fighter is punched and:
a) a part of his/her body other than the feet touch the canvas (such as his butt or glove)
b) when a fighter is being held up by the ropes (i.e. the fighter would have fallen had the ropes not been there)
c) when a fighter is hanging on, through or over the ropes and
d) cannot protect himself and, for whatever reason, is lodged on a position where he can't fall
A slip or fall to the canvas by a boxer, resulting in any of the above conditions, is not a knockdown.
Note: The Association of Boxing Commissions Referee Rules and Guidelines state: "Before a bout can resume after a boxer has been knocked down, fallen or slipped to the floor of the ring, the Referee shall wipe his gloves." The referee wipes the gloves on his shirt to remove any dirt or debris that might have gotten on the fighter's gloves from the canvas.
Association of Boxing Commissions Referee Rules and Guidelines