Ăl'tern, a.

a. [Lat. alternus, contracted from alterinus, from alter, another; Fr. alterne.]

1. Acting by turns; one succeeding another; alternate. [Obs.] Milton.
2. (Crystallog.) Exhibiting, on two parts, an upper and a lower part, faces which alternate among themselves, but which, when the two parts are compared, correspond with each other. Cleveland.

Altern base (Trig.), a term used in distinction from the true base. Encyc. Brit.