A vesicle-mediated transport process in which cells take up external materials or membrane constituents by the invagination of a part of the plasma membrane to form a new membrane-bounded vesicle. [ http://openisbn.com/search.php?q=0716731363&isbn=1/ http://openisbn.com/search.php?q=0198506732&isbn=1/ http://en.wikipedia.org/wiki/Endocytosis GOC:mah ]