mid-15c., from Old French alternacion "alternation," from Latin alternationem (nominative alternatio) "an interchanging," noun of action from past-participle stem of alternare "to do first one thing then the other; exchange parts," from alternus "one after the other, alternate, in turns, reciprocal," from alter "the other" (see alter).
"apparent displacement of an object observed, due to an actual displacement of the observer," 1570s, from French parallaxe (mid-16c.), from Greek parallaxis "change, alteration, inclination of two lines meeting at an angle," from parallassein "to alter, make things alternate," from para- (see para- (1)) + allassein "to change," from allos "other" (from PIE root *al- "beyond"). Related: Parallactic.
early 15c., quarterli, "four times a year, once a quarter," from quarter (n.1) + -ly (2). As an adjective from mid-15c., "occupying alternate quarters" (of a coat of arms), with -ly (1). As a noun, "a quarterly publication," from 1830, from the adjective. Earlier the adverb was used in a now-obsolete sense of "into quarters" (c. 1400).