Gap junction: An organized collections of protein channels in cell membranes that allows ions and small molecules to pass between adjacent cells. The protein channels that make up gap junctions consist of two connexons. One connexon resides in the membrane of one cell. It aligns and joins the connexon of the neighboring cell, forming a continuous aqueous pathway by which ions and small molecules can freely pass (passively) from one cell to the other.

Each connexon consist of six subunits called connexins. The connexin genes have been highly conserved during evolution. In some cells the connexons are formed of six identical connexins or of some combination of two different connexins.

Gap junctions exist in almost all cell types., some exceptions being skeletal muscle, red blood cells and circulating blood lymphocytes. The size and number of gap junctions per cell can change, depending on such factors as the point in the cell cycle and exposure of the cell to environmental stress.