In a
liquid crystal the
mesogen is the portion of the molecule which causes the
mesophase to form. Traditionally a
liquid crystal molecule is composed of
rigid and
flexible sections. The
mesogen is the rigid portion.
The presence of the mesogen means that the molecules are forced to arrange themselves with some long-range orientational and / or positional order in one, two or three dimensions. This order distinguishes the liquid crystal from the disordered liquid.