单轴晶体是一种透射光学组件，此组件其中某一个结晶轴的折射率不同于其他两个结晶轴，即 。这个独特的轴被称为异常轴，同时也被称为光轴。当晶体可呈现一个单一的光轴时，在这种情况下，此晶体为单轴。单轴晶体是由一种各向异性的均匀介质构成，其在不同的方向所显示的电磁特性不同。这些晶体一般是非磁性介质，因此在不同方向所展现的不同材料特性是介电常数，而不是磁导率。

当光传播通过具有最小折射率的轴时，拥有更高的相速度，则该轴被称为快轴。同样地，具有最高折射率的轴被称作慢轴，因为光沿着该轴传播的相速度是最低的。光轴可以是晶体的快轴或慢轴，其区分条件取决于材料特性。单轴晶体可以区分为负单轴晶体及正单轴晶体两种。负单轴晶体（如方解石 CaCO₃，红宝石Al2O3）具有折射率 ，所以对于这些晶体，异常轴（光轴）为快轴。而对于正单轴晶体（如石英 SiO₂，氟镁石（氟化镁）MgF₂，金红石 TiO₂），具有折射率 ，因此异常轴（光轴）为慢轴。

Uniaxial crystals are transmissive optical elements in which the refractive index of one crystal axis is different from the other two crystal axes (i.e. n_i ≠ n_j = n_k). This unique axis is called the extraordinary axis and is also referred to as the optic axis. Light travels with a higher phase velocity through an axis that has the smallest refractive index and this axis is called the fast axis. Similarly, an axis which has the highest refractive index is called a slow axis since the phase velocity of light is the lowest along this axis. The optic axis can be the fast or the slow axis for the crystal depending upon the material. Negative uniaxial crystals (e.g. calcite CaCO₃, ruby Al₂O₃) have n_e < n_o so for these crystals, the extraordinary axis (optic axis) is the fast axis whereas for positive uniaxial crystals (e.g. quartz SiO₂, sellaite (magnesium fluoride) MgF₂, rutile TiO₂), n_e > n _o and thus the extraordinary axis (optic axis) is the slow axis.