弦论与相关的超重力理论中，膜（英语：brane）为一物理实体，将点粒子的概念推广至更高维度。举例来说，点粒子可以视为零维的膜，而弦则可视为一维的膜；更高维的膜也可能存在。在p维度的情形，这些膜称为p膜。膜的英文本brane源于另个英文本membrane，后者指的是二维膜。

膜是动力学物体，在时空中行进，所根据的是量子力学的规则。它们带有质量与其他性质，例如电荷。一个p膜的行进在时空中扫出了(p+1)维度的体积，称之为世界体积（worldvolume）。物理学家研究类似于电磁场的场物理，这些场存在于膜的世界体积。

In string theory and related theories such as supergravity theories, a brane is a physical object that generalizes the notion of a point particle to higher dimensions. For example, a point particle can be viewed as a brane of dimension zero, while a string can be viewed as a brane of dimension one. It is also possible to consider higher-dimensional branes. In dimension p, these are called p-branes. The word "brane" comes from the word "membrane" which refers to a two-dimensional brane.