= Liquid crystal
{wiki}
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.
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