In biology, caveolae, which are a special type of lipid raft, are small invaginations of the plasma membrane in the cells of many vertebrates. Wikipedia

A membrane raft that forms small pit, depression, or invagination that communicates with the outside of a cell and extends inward, indenting the cytoplasm and the cell membrane. Examples include flask-shaped invaginations of the plasma membrane in ad Wikidata

A small (50–100 nanometer) invagination of the plasma membrane in many vertebrate cell types. Wiktionary

Small invagination of the plasma membrane. Wiktionary (translation)