nounhistorical, rare Chemistry
An element or radical whose valency is represented by an even number. Contrasted with "perissad".
Mid 19th century (in an earlier sense). From ancient Greek ἄρτιος (of numbers) even (from ἄρτι just, exactly from the same Indo-European base as classical Latin ars, Armenian ard recently, Lithuanian artus (adjective), arti (adverb) near) + -ad.