Confused about audio speak? Here are some terms I keep using, it's a work in progress.
In a typical domestic sized room, the bass response is lumpy - it has peaks and dips which are the evidence of "room modes." They are points in the room where the bass is stuffed.
This is the point above which room modes are spaced so close together that they smooth out and the room no longer causes irregular peaks and dips. This usually occurs around 200 Hz.