Definitions for "Bounce flash"
Here light from the flash is aimed at a wall or ceiling to bounce, i.e. reflect onto the subject.
It's a technique of "bouncing" the light from a flash unit off of a reflector. This is one of the ways to get softer, more even lighting on your subject. The "reflector" can be just about anything, a wall, a ceiling, a piece of cardboard (light colored would be best), or cloth.
To bounce the light from a flash unit on a wall, ceiling or similar and then onto the photographed subject. This is often done to soften shadows and get a more flattering light.