The Collaborative International Dictionary of English v.0.48:

Additional \Ad*di"tion*al\, a.
   Added; supplemental; in the way of an addition.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Additional \Ad*di"tion*al\, n.
   Something added. [R.] --Bacon.
   [1913 Webster]

WordNet (r) 3.0 (2006):

additional
    adj 1: further or added; "called for additional troops"; "need
           extra help"; "an extra pair of shoes" [syn: extra,
           additional]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "additional":
   accessory, accidental, accumulative, added, additive, additory,
   adscititious, adventitious, ancillary, another, appurtenant,
   ascititious, auxiliary, casual, circumstantial, collateral,
   contingent, contributory, cumulative, else, extra, farther,
   fortuitous, fresh, further, incidental, inessential, more, new,
   nonessential, other, plus, renewed, secondary, spare, subsidiary,
   summational, summative, superadded, superfluous, supernumerary,
   supervenient, supplemental, supplementary, surplus, ulterior,
   unessential