What is the X in X-mas?
late 14c., "placed on the other side of (something)," from Old French oposite "opposite, contrary" (13c.), from Latin oppositus "standing against, opposed, opposite," past participle of opponere "set against" (see opponent). Meaning "contrary in nature or character" is from 1570s. As a noun from late 14c. As a preposition from 1758. As an adverb from 1817. Related: Oppositely.
Arranged as one of a pair on either side of a stem or twig. Maple and ash trees have opposite leaves. Compare alternate.