1. (a.) Not essential.

2. (n.) A thing not essential.

accessory accident accidental addendum addition additional adjunct adrift adscititious adventitious aleatory appendage appurtenance appurtenant ascititious auxiliary back-burner beside the mark beside the point beside the question casual circumstantial collateral conditional contingency contingent de trop dinky dispensable excess expendable expletive extra extraneous extrinsic fortuitous gratuitous happenstance immaterial impertinent in excess inadmissible inapplicable inapposite inappreciable inappropriate incidental inconsequent inconsequential inconsiderable inessential inferior insignificant irrelative irrelevant little mere chance minor minute needless negligible nihil ad rem not at issue not vital not-self occasional off the subject other out-of-the-way parenthetical petit pleonastic prolix provisional redundant secondary small spare subsidiary superadded superaddition supererogatory superfluous supervenient supplement supplemental supplementary tautologic tautologous technical to spare uncalled-for unessential unimpressive unnecessary unneeded unnoteworthy unrequired verbose