场线是由矢量场和初始点设置的轨迹。在空间里，矢量场在每一个位置，都设置了一个方向。只要按照矢量场在每一个位置所指的方向来追踪路径，就可以素描出正确的场线。更精确地说，场线在每一个位置的切线必须平行于矢量场在那一个位置的方向。

在空间内，由于，伴随着每一个点的矢量，组合起来，构成了矢量场，场线可以说是一个专为矢量场精心打造的显像工具，能够清楚地显示出矢量场在每一个位置的方向。假若矢量场描述的是一个速度场，则场线跟随的是流体的流线。在磁铁的四周洒散铁粉，可以清楚地显示出磁场的磁场线。静电荷的场线称为电场线，从正电荷往外扩散，朝着负电荷聚集。

A field line is a locus that is defined by a vector field and a starting location within the field. Field lines are useful for visualizing vector fields, which are otherwise hard to depict. Note that, like longitude and latitude lines on a globe, or topographic lines on a topographic map, these lines are not physical lines that are actually present at certain locations; they are merely visualization tools.