I meet such expression often when working on 'drainage' related documents at the ministry of Housing and urban Development

Wherever there is an obstacle in a fluid stream, vortices will form. You can see these on the downstream part of bridge piers (for example), where a part of the stream is actually going backwards. These will definitely occur at any abrupt change in cross-section of a pipe, but can be avoided by using a slowly divergent pipe (as stated here), known as a diffuser.

No context but from your other questions , I think you are talking
about pipes etc
IF AND ONLY IF this is the case,' backflow' is probably best as some of the other terms are used more for rivers and other natural systems