Combat Maneuver
You can attempt to trip your opponent in place of a melee attack. If your attack exceeds the target’s CMD, the target is knocked prone. If your attack fails by 10 or more, you are knocked prone instead. If the target has more than two legs, add +2 to the CMD to resist trips for each additional leg it has. Some creatures—such as oozes, creatures without legs, and flying creatures—cannot be tripped.
Weapon Trait
Trip: When you use a trip weapon to make a trip attack, if you are tripped during your own trip attempt, you can drop the weapon to avoid being tripped.
Universal Monster Reference
Trip (Ex) A creature with the trip special attack can attempt to trip its opponent as a free action without provoking an attack of opportunity if it hits with the specified attack. If the attempt fails, the creature is not tripped in return.
Format: trip (bite); Location: individual attacks.