A small tea room.
Yojohan (4.5 mat) and smaller are considered koma.
A more precise definition would be all rooms that do not have a fumikomi tatami. A fumikomi tatami is the first Tatami you step onto when you enter the room from Sadoguchi. Look at Chashitsu for more information.
In koma Tana is not used unless it is a fixed tana that is a permanent part of the room.