The process by which materials enter a cell without passing through the plasma membrane. The membrane folds around material outside the cell, resulting in the formation of a saclike vesicle into which the material is incorporated. This vesicle is then pinched off from the cell surface so that it lies within the cell (see endosome). Both phagocytosis and pinocytosis are forms of endocytosis. In receptor-mediated endocytosis, cells selectively take in substances (e.g. hormones, low-density lipoproteins) that bind to receptors on the cell surface. Compare exocytosis.