efficacy

noun2

1. •
   The ability to produce a desired effect under ideal testing conditions.

   • […]and this hath even made me suspect the efficacy of reliques, to examine the bones, question the habits and appurtenances of Saints, and even of Christ himself. — 1642
2. •
   A degree of ability to produce a desired effect; effectiveness.

   • Toothbrush with improved efficacy — 1996