A room in which the walls, ceiling and floor reflect most of the sound. To accomplish this, they are made of a very hard material (such as, say, concrete). To spread the reflected sound as much as possible, curved reflectors are often hung in such rooms.
Like an anechoic chamber it is used to conduct experiments and measurements in acoustics and related sciences. A typical measurement that is accomodated by a reverberation room, is to measure the total generated sonic energy produced by a loudspeaker. A speaker doesn't give off an equal amount of energy in all directions, so setting up test microphones in lots of different points around the speaker and averaging the measurements would be impractical. Instead, you put it in a reverberation room, and one or a few measurements will often suffice. This is because the sound is reflected in all directions, and the total sound is (in theory) equally strong in all points in the room.
Another typical use of a reverberation room is to measure the sound attenuation in walls and panels. This is done by comparing the reflected energy of a known sound source before and after covering part of the walls, ceiling or floor with it.