septic tank

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A sewage-disposal tank in which a continuous flow of waste material is decomposed by anaerobic bacteria.

from The Century Dictionary.

• noun See tank.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A small-scale watertight treatment system for domestic sewage in which the flow is slowed to allow sedimentation and sludge digestion by bacteria to take place.
• noun Cockney rhyming slang, Australian rhyming slang Yank (American person)

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun large tank where solid matter or sewage is disintegrated by bacteria