A liposome is a spherical vesicle with a membrane composed of a phospholipid bilayer used to deliver drugs or genetic material into a cell.

The lipid bilayer can fuse with other bilayers (e.g., the cell membrane), thus delivering the liposome contents. By making liposomes in a solution of DNA or drugs,(which would normally be unable to diffuse through the membrane), they can be (indiscriminately) delivered past the lipid bilayer.