The Collaborative International Dictionary of English v.0.48:

optimal \op"ti*mal\, a.
   Best possible; most desirable; optimum; as, the optimal
   concentration of a drug.
   [PJC]

WordNet (r) 3.0 (2006):

optimal
    adj 1: most desirable possible under a restriction expressed or
           implied; "an optimum return on capital"; "optimal
           concentration of a drug" [syn: optimum, optimal]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "optimal":
   best, champion, choice, elect, elite, for the best, greatest,
   handpicked, matchless, optimum, paramount, peerless, picked, prime,
   prize, quintessential, select, supreme, surpassing, unmatchable,
   unmatched, unparalleled, unsurpassed, very best



The Free On-line Dictionary of Computing (30 December 2018):

optimal

   1.  Describes a solution to a problem which
   minimises some cost function.  Linear programming is one
   technique used to discover the optimal solution to certain
   problems.

   2.  Of code: best or most efficient in time,
   space or code size.

   (1995-10-05)