A temporary protrusion on the cell surface. Initially the cell extends a membrane process known as a lamellopodium. This is accompanied by controlled polymerization of actin filaments at the leading edge and the subsequent incorporation of these actin filaments into bundles and networks. The origin of the actual force that propels the cell forward is unknown, although it is thought to be the polymerization of the actin filaments. Amoeba and Acantharea use pseudopodia.