A small pocket, vesicle, cave, or recess communicating with the outside of a cell and extending inward, indenting the cytoplasm and the cell membrane. Such caveolae may be pinched off to form free vesicles within the cytoplasm. They are considered to be sites of uptake of materials into the cell, expulsion of materials from the cell, or sites of addition or removal of cell (unit) membrane to or from the cell surface. [ ]