A liquid crystal is a phase of matter that lies between the disorder of liquids and the neatly arranged lattices of crystals.
Unlike regular liquids, liquid crystals must have at least one degree of macroscopic order, even if it is only the angular alignment of a nematic. Liquid crystals lack the full three dimensional position and orientation order of ordinary crystals.
In the isotropic phase, the particles are at random positions and orientations. In the nematic phase, the particles have an overall direction of orientation, but the positions of the individual particles are still random. The direction of alignment is called the director. The nematic order is not perfect and there can be considerable fluctuation about the director. A cholesteric is similar to a nematic, but in a certain direction the director twists about in a helix of a given pitch. In smectics, the particles form a set of parallel planes of regular spacing. Within a given plane of a smectic liquid crystal, the order is liquid-like. Several types of smectics have been found. In the columnar phase, the particles have liquid order within a regular two-dimensional lattice of columns; usually the lattice is hexagonal.
Although many liquid crystals are composed of only one kind of molecule, they need not be. The main property needed is one dimension much different from another. The particles can be long rod-like objects or flat disks.
While some liquid crystals consist of just one chemical, a great number are mixtures. When the multicomponent property is crucial to a liquid crystal's existence, then this system is referred to as lyotropic.