The Collaborative International Dictionary of English v.0.48:

Reactive \Re*act"ive\ (r[-e]*[a^]kt"[i^]v), a. [Cf. F.
   r['e]actif.]
   Having power to react; tending to reaction; of the nature of
   reaction. -- Re*act"ive*ly, adv. -- Re*act"ive*ness, n.
   [1913 Webster]