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Coelomia or coelom (Latin coelomum, from Greek κοίλωμα - recess, cavity) is a secondary coelomia, which is a system of communicating cavities that arises as a result of incomplete division of the primary coelomia into segments in some types of multicellular animals.

Coelomia is one of the main characters of the phylum Metazoa and is found in a variety of animal species, including flatworms, roundworms, ciliated worms, molluscs, echinoderms, starfish, coelenterates, sponges, archaeocyaths, ctenophores, and others. Coelomia is characteristic of most representatives of the phylum Spongozoa, as well as some representatives of the phylum Cnidaria and phylum Porifera.

The occurrence of coelomia is associated with the process of evolution, which allowed multicellular animals to take advantage of the coelomic blood circulation.

In the body of many animals there is a coelom, which is a secondary body cavity. Coelomia is a special method of gastrulation in which invaginations of the blastura and germ layer (ectoderm and endoderm, respectively) and the adjacent mesodermal layer form a body cavity - the coelom. Loss of two processes of metagenesis (doubling and splitting of the primary body cavity). Sometimes coelomia leads to death as a result of the impossibility of correct orientation in space. The meaning of coelomia is that the whole organism develops faster, which is prevented by the release of tissue into the body cavity