What is the definition or description of: knee pop?

Pop and pain: The symptom of popping or snapping in the knee can be a sign of a few different problems. One of the key distinguishing factors is whether or not the popping or snapping causes or is associated with pain. Popping or snapping not associated with painful symptoms is often not a sign of a significant problem.

A noise or feeling: It can be normal ( in the case of a plica) or significant if an injury occurs (acl). If it is associated with pain, then it needs to be checked.