Brane

In theoretical physics, branes or p-branes are spatially extended objects that appear in string theory and its relatives (M-theory and brane cosmology). The variable p refers to the spatial dimension of the brane. That is, a 0-brane is a zero-dimensional particle, a 1-brane is a string, a 2-brane is a "membrane", etc. Every p-brane sweeps out a (p+1)-dimensional world volume as it propagates through spacetime.

Originally string theory was a theory of 1-branes called strings. By the mid-1990s it became apparent that the theory could be extended to also include higher dimensional objects. Typically these objects are non-perturbative features of the theory (meaning they do not appear in perturbation theory) which is partially why early string theorists were unaware of them.