An Etymological Dictionary of Astronomy and Astrophysics

Next after the second; the ordinal number for three. → Newton’s third law of motion; → third contact; → third dredge-up; → third law of thermodynamics.

Etymology (EN): M.E. thirde, O.E. (north) thirda, variant of ridda, from P.Gmc. *thridjas (cf. O.Fris. thredda, O.S. thriddio, M.L.G. drudde, Du. derde, O.H.G. dritto, Ger. dritte, Goth. thridja).

Etymology (PE): Sevom, ordinal number for sé, → three.