Source : Webster's Revised Unabridged Dictionary (1913)

Effectiveness \Ef*fect"ive*ness\, n.
   The quality of being effective.

Source : WordNet®

effectiveness
     n 1: power to be effective; the quality of being able to bring
          about an effect [syn: {effectivity}, {effectualness}, {effectuality}]
          [ant: {ineffectiveness}]
     2: capacity to produce strong physiological or chemical
        effects; "the toxin's potency"; "the strength of the
        drinks" [syn: {potency}, {strength}]