Fatty acid (Chem.), any one of the paraffin series of monocarbonic acids, as formic acid, acetic, etc.; -- so called because the higher members, as stearic and palmitic acids, occur in the natural fats, and are themselves fatlike substances. -- Fatty clays. See under Clay. -- Fatty degeneration (Med.), a diseased condition, in which the oil globules, naturally present in certain organs, are so multiplied as gradually to destroy and replace the efficient parts of these organs. -- Fatty heart, Fatty liver, etc. (Med.), a heart, liver, etc., which have been the subjects of fatty degeneration or infiltration. -- Fatty infiltration (Med.), a condition in which there is an excessive accumulation of fat in an organ, without destruction of any essential parts of the latter. -- Fatty tumor (Med.), a tumor consisting of fatty or adipose tissue; lipoma.