Refer to figure.
According to EASA, a bowl exists where mountains surround a confined area (often formed by a small lake or stream) with open access on one side into a valley.

It's important to understand that the surrounding walls of a mountain bowl can be steep and high, with limited escape options. This technique demands a high degree of skill and should only be attempted by a proficient pilot.

An approach can normally be made by entering the bowl downwind, flying around the sides of the bowl, and then descending into the wind to a flat area close to the bowl's exit.