Uniaxial crystal - Fig crystal

...In an optically anisotropic medium, light is split into two linearly polarized (polarized) beams with mutually orthogonal vibration directions, each of which travels through the crystal at different speeds.
[Uniaxial crystals, biaxial crystals]
However, even in optically anisotropic solids, birefringence does not occur if light is incident from a specific direction. This direction is called the optical axis, and a crystal with only one optical axis is called a uniaxial crystal, and a crystal with two optical axes is called a biaxial crystal.

*Some of the terminology that refers to "uniaxial crystal" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…また，光学的異方体の内部では，光は互いに直交する振動方向をもつ二つの直線偏光(偏光)に分かれ，それぞれ結晶中を異なる速さで進む。
［1軸性結晶，2軸性結晶］
　しかし，光学的異方体でも，特定の方向から光が入射すると複屈折現象は現れない。この方向を光軸と呼び，光軸を一つしかもたないものを1軸性結晶，二つもつものを2軸性結晶と呼ぶ。…

※「１軸性結晶」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報