平行是一个几何学术语。在平面几何中，永远不会相交的多条直线，或者多个平面彼此互相平行。在欧几里得几何中，由平行公设，平面上过直线外一点恰好可以作一条与它平行的直线。在非欧几何中，根据空间曲率的不同，过直线外一点可以作多条或零条与它平行的直线。

在三维空间或一般的欧几里得空间中，直线或平面的平行关系视乎其方向矢量或法矢量，但与二维平面一样，过直线外一点也只能作一条与它平行的直线，并且过平面外一点也只能作一个与它平行的平面。然而，过平面外一点可以作无数条与之平行的直线（都在唯一的与它平行的平面上）。

In geometry, parallel lines are lines in a plane which do not meet; that is, two lines in a plane that do not intersect or touch each other at any point are said to be parallel. By extension, a line and a plane, or two planes, in three-dimensional Euclidean space that do not share a point are said to be parallel. However, two lines in three-dimensional space which do not meet must be in a common plane to be considered parallel; otherwise they are called skew lines. Parallel planes are planes in the same three-dimensional space that never meet.