Quadrupedalism

Quadrupedalism is a form of land animal locomotion using four limbs or legs.

Quadrupeds vs. tetrapods
Not all four-limbed animals are quadrupeds. Although arms and wings are, in the evolutionary sense, modified legs, four-limbed animals are in fact classed as tetrapods, members of the taxonomic unit Tetrapoda. These include all vertebrates with quadrupedal ancestors, including mammals, reptiles, amphibians, and birds.

The distinction between quadrupeds and tetrapods is important in evolutionary biology, particularly in the context of bipeds, winged animals, and animals whose limbs have adapted to other roles (e.g. fins, in the case of cetaceans and pinnipeds). All of these animals are tetrapods, but none are quadrupeds. Even snakes, whose limbs have become entirely vestigial, are nevertheless tetrapods.