Aliphatic \Al`i*phat"ic\, a. [Gr. ?, ?, oil, fat.] (Org. Chem.) Of, pertaining to, or derived from, fat; fatty; -- applied to compounds having an open-chain structure. The aliphatic compounds thus include not only the fatty acids and other derivatives of the paraffin hydrocarbons, but also unsaturated compounds, as the ethylene and acetylene series. Compare {alicyclic} and {aromatic}. [Webster 1913 Suppl.]