(a.) Added to supply what is needed; additional; being, or serving as, a supplement; as, a supplemental law\; a supplementary sheet or volume.

accessory accidental added additional adjunct adscititious adventitious ancillary annexed another appended appurtenant ascititious attached auxiliary casual circumstantial collateral contingent contributory excess extra extraneous farther fortuitous fresh further incidental inessential more new nonessential other plus secondary spare subordinate subsidiary superadded superfluous supernumerary supervenient supplemental supportive surplus ulterior unessential