1) Connections between cells which allow passage of small molecules and electric current. Gap junctions were first described anatomically as regions of close apposition between cells with a narrow (1-2 nm) gap between cell membranes. The variety in the properties of gap junctions is reflected in the number of connexins, the family of proteins which form the junctions.

2) An area of contact between adjacent cells characterized by modification of the plasma membranes for intercellular communication or transfer of low molecular-weight substances.