Source : Webster's Revised Unabridged Dictionary (1913)

Reactive \Re*act`ive\, 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.

Source : WordNet®

reactive
     adj 1: participating readily in reactions; "sodium is a reactive
            metal"; "free radicals are very reactive" [ant: {unreactive}]
     2: tending to react to a stimulus; "the skin of the geriatric
        is less reactive than that of younger persons"- Louis Tuft