Lipid bilayer either surrounding cytoplasm or, more broadly, also those lipid bilayers found suspended within cytoplasm.
Prominent examples of cell membranes are the plasma membrane (a.k.a., ) and the various membranes associated with the endomembrane system of eukaryotic cells. See also the concept of .
It is possible to equate the cell membrane simply with a cell's plasma membrane as in the cell membrane. In doing so, however, it becomes difficult to describe a cell's membranes collectively (e.g., such as indcluding endomembranes).
Similarly, how then would one describe the outer membrane of Gram-negative bacteria, which clearly is a cell membrane though not also a plasma membrane? In any case, note that cell walls are unquestionably not cell membranes nor components of cell membranes, though they are components of cell envelopes.
Cell membranes are a subset of and other kinds of membranes additionally exist. One example is the found in membrane filters or .
In terms of strictly biological membranes, are a form of membrane that is neither a cell membrane nor a lipid bilayer but instead consists of extrecellular that in fact does not surround cells. See too mucous membrane.