In biology, motility is the ability to move spontaneously and actively, consuming energy in the process. Most animals are motile but the term applies to unicellular and simple multicellular organisms, as well as to some mechanisms of fluid flow in multicellular organs, in addition to animal locomotion. Motile marine animals are commonly called free-swimming. The opposite of motility is sessility. Motility may also refer to an organism’s ability to move food through its digestive tract, i.e., peristaltics (gut motility, intestinal motility, etc.).