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A liquid crystal has some molecular order while retaining liquid-like motion. The distinction between orientational and positional order separates its main phases.

A nematic liquid crystal has long-range orientational order about a nematic director $\mathbf{n}$, with $\mathbf{n}$ and $-\mathbf{n}$ equivalent for apolar molecules, but no periodic density lattice. Its nematic order parameter is $S=⟨(3{\cos }^2\theta -1)/2⟩$, where $\theta$ measures molecular alignment with the director. A cholesteric liquid crystal is a chiral nematic whose director rotates in space, producing a helical pitch while remaining locally nematic.

A smectic phase adds a periodic density modulation in one direction, forming layers with liquid-like motion within each layer. In a smectic A phase, the director is perpendicular to the layers; in a smectic C phase, it is tilted relative to the layer normal. A columnar liquid crystal instead has columns with a two-dimensional positional lattice and comparatively fluid motion along them.

These orders have different experimental signatures. Birefringence under crossed polarizers reveals orientational anisotropy, while X-ray diffraction can detect a smectic layer spacing $d$ through a peak near $q=2\pi /d$. Optical anisotropy alone does not establish a layer lattice.

A smectic A phase has layered positional order with the average molecular orientation along the layer normal. Molecular positions within each layer remain liquid-like. Its orientational axis is described by a nematic director.

A smectic phase is a layered liquid crystal with periodic density or composition variation along one direction and fluid-like positional organization within a layer. A simple order-parameter profile is $\varphi (\mathbf{r})=A\cos ({\mathbf{q}}_0\cdot \mathbf{r})$, with layer spacing $2\pi /|{\mathbf{q}}_0|$.

Layer displacement and orientational fluctuations are additional degrees of freedom, so a fixed cosine is only the simplest mean profile. A smectic A phase has average molecular alignment along the layer normal; a smectic C phase has a tilted director. Scattering can reveal the preferred layer spacing and orientation through peaks near ${\mathbf{q}}_0$. The layer structure does not imply a full three-dimensional crystal lattice of fixed molecular positions.