Source : WordNet®

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

Source : Free On-Line Dictionary of Computing

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)