An Etymological Dictionary of Astronomy and Astrophysics

1. Happening or following in turns; succeeding each other continuously.
   

2a) (v.intr.) To occur in a successive manner (day alternates with night); To change back and forth from one state, action, or place to another.

2b) Electricity: To reverse direction at regular intervals in a → circuit.

3. (v.tr.) To do or execute in succession or one after another (to alternate work and rest).

Etymology (EN): From L. alternus “one after the other,” p.p. of alternare “to do first one thing, then the other,” from alternus “every other,” from alter “the other”.

Etymology (PE): 1) Peyvâr from pey “step; after” (as in peyâpey “successively, repeatedly”) + Pers. vâr “turn (succession), time (repetition), alternation”. Compare with Skt. vâra “one’s turn, appointed time, alteration, succession”.

2, 3) Peyvâr šodan, peyvâr kardan, peyvâridan, verbs from peyvâr “alternate,” as above.