Butyric Acid (aka Butanoic Acid) is a saturated fatty acid found naturally in butter, and is normally bound up in glycerides.
When Butyric Acid is freed up in butter, it’s what causes the rancid smell. It is also present in vomit, Gingko fruit and foot sweat.
It’s formula is CH3CH2CH2-COOH.
The word comes from the Latin word for butter, butyrum.