SRD under Cleave said:Benefit
If you deal a creature enough damage to make it drop (typically by dropping it to below 0 hit points or killing it), you get an immediate, extra melee attack against another creature within reach. You cannot take a 5-foot step before making this extra attack. The extra attack is with the same weapon and at the same bonus as the attack that dropped the previous creature. You can use this ability once per round.
Goldmoon said:I have always defined drop (and seen it defined in d&d) as falling down or making someone fall down.
Mistwell said:Except the rule isn't "make them drop", as in "fall down", it's "If you deal a creature enough damage to make it drop (typically by dropping it to below 0 hit points or killing it)", as in "go unconscious or kill".
ThirdWizard said:However, there is an interesting note with the Knockdown feat.