Airborne

Airborne creatures are capable of operating full-time or part-time without touching the ground.

That includes:
 * creatures that fly with wings
 * creatures that float above the ground by being lighter than air
 * creatures that lift themselves up, levitate and propel themselves around by magic or psionic means
 * some incorporeal creatures that have no physical body to weigh them down

Some airborne monsters are actually hovering just above the ground, and if you use the environment to your advantage, they'll have the same pathing difficulties reaching you that monsters bound to the ground have; other airborne monsters really fly and can chase you in a straight line.

Airborne monsters can't be tripped, since their lower limbs are not in contact with the ground (assuming they do have lower limbs). They can still be knocked down by other effects; for example, a winged monster that suddenly falls asleep usually falls to the ground as well since it stops flapping its wings.