one tatami mat
Refers to a single tatami mat, often used when describing room size or area in terms of tatami mats.
This closet is about the size of one tatami mat.
I placed a small desk in a one-tatami-mat space.
一畳 is the unit for a single tatami mat used to measure area, whereas 四畳半 refers to a specific room size (or the room itself) of four-and-a-half mats.
Compound of 一 (one) and 畳 (tatami mat).