Animals possessing one or more body cavities that are found entirely within mesodermal tissue.
All coelomates are triploblastic eumetazoans. Their coeloms can be employed in within the animal (contrasting, for example, the use of a gastrovascular cavity).
Coeloms also allow the shape of body organs to minimally impact the shape of the animal, or more generally its overall form. You are at best barely aware of the when simply looking at an animal and this is because almost all of that motion does not impact the exterior of the animal. It instead occurs within a body cavity, a.k.a., a coelom.
Contrast coelomates, which make up the majority of , with acoelomates, though see also pseudocoelomates.