A small static flask-shaped depression in the plasma membrane of a cell that is lined by transmembrane proteins called caveolins and contains receptors for various extracellular signalling molecules. There is a two-way traffic in and out of the cell via caveolae, and caveolin-containing vesicles are transported from within the cell to the caveolar membrane along microtubules. Caveolae are also involved in endocytosis and play an important role in the cellular uptake of bacterial toxins, viruses, and proteins.