sitz bath

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A bathtub shaped like a chair in which one bathes in a sitting position, immersing only the hips and buttocks.
2. n. A bath taken in such a tub especially for therapeutic reasons.

Century Dictionary and Cyclopedia

1. n. Same as hip-bath.
2. n. A tub of wood, metal, etc., adapted for such a bath.

Wiktionary

1. n. A hydrotherapeutic bath taken in a sitting position.

GNU Webster's 1913

1. A tub in which one bathes in a sitting posture; also, a bath so taken; a hip bath.

WordNet 3.0

1. n. a bathtub in which your buttocks and hips are immersed as if you were sitting in a chair and you bathe in a sitting position