A liquid crystal is a phase with orientational or partial positional order while retaining some fluid-like motion. Its structure lies between a fully disordered isotropic fluid and a crystal with positional order in all spatial directions.
In a nematic liquid crystal, molecular axes prefer a common unoriented nematic director without forming a three-dimensional lattice of fixed positions. The nematic order parameter measures that alignment and can vary across the sample. Orientational distortions cost elastic energy and can form topological defects. Applied fields and boundary anchoring can change the director pattern, thereby changing optical propagation. The phase should be specified, since different liquid-crystal phases break different symmetries and need different order parameters.
Liquid crystals are a state of matter that have properties between those of conventional liquids and solid crystals. In a solid crystal, the molecules are arranged in an ordered structure, while in a conventional liquid, they are disordered and free to move around. Liquid crystals, however, exhibit a unique combination of both order and fluidity.
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