A knockdown occurs when a boxer, after being hit by his opponent: 1) touches the ring floor with some part of his body other than the feet; 2) has any part of his body outside the ropes; 3) is hanging on the ropes helplessly; or 4) is judged to be in a semi-conscious state and unable to continue fighting.
A slip or fall on the canvas by a boxer, resulting in any of the above conditions, is not a knockdown.
Note: You will often see the referee wipe the gloves of a knockdowned boxer on the front of his shirt, once the boxer regains his feet. This is to remove any resin from the canvas floor that may have gotten on the gloves. If resin is left on a glove, it can damage the opponent's eyes during continued fighting.