Loosely speaking, it's a structure that can't be continuously transformed away. For example, consider a (frictionless, non-dissipative etc.) fluid flowing in a

Loosely speaking, it's a structure that can't be continuously transformed away. For example, consider a (frictionless, non-dissipative etc.) fluid flowing in a circle around a given point: a vortex. Since this has net angular momentum, and a fluid moving uniformly in one direction doesn't, it's impossible for the vortex to dissipate. We can move it around, or split it up, or fuse it with other vortices, but we can't just smooth it out.