What is "facultative biped"

n. (context biology English) Adapted primarily for walking on two legs, but able to walk on four legs.

A facultative biped is an animal that is capable of walking or running on two legs, often for only a limited period, in spite of normally walking or running on four limbs or more. Well-known examples include many lizards such as the Basilisk lizard, and even some cockroaches when running at top speed. Low-speed facultative bipedality is less common; the gibbon, a primate with an anatomy highly specialized for arboreal locomotion, can walk bipedally in trees or on the ground with their arms raised for balance.

In order to be considered a true facultative biped, an animal must be capable of sustained movement over many strides while bipedal—simply adopting a static bipedal posture while resting or looking around is not sufficient.