叶理（Foliation）是在岩石中的纤维状平面组织，常作为变质岩鉴定特征的依据。

岩石受到应力作用后，内部的矿物会有部份压溶的现象。当这些矿物再结晶，则矿物会沿着垂直主轴应力作用的面生长，成为纤维状的组织，即为叶理。

Foliation in geology refers to repetitive layering in metamorphic rocks. Each layer may be as thin as a sheet of paper, or over a meter in thickness. The word comes from the Latin folium, meaning "leaf", and refers to the sheet-like planar structure. It is caused by shearing forces (pressures pushing different sections of the rock in different directions), or differential pressure (higher pressure from one direction than in others). The layers form parallel to the direction of the shear, or perpendicular to the direction of higher pressure. Nonfoliated metamorphic rocks are typically formed in the absence of significant differential pressure or sheer. Foliation is common in rocks affected by the regional metamorphic compression typical of areas of mountain belt formation (orogenic belts).