reactive

from WordNet (r) 3.0 (2006)

reactive
    adj 1: participating readily in reactions; "sodium is a reactive
           metal"; "free radicals are very reactive" [ant:
           {unreactive}]
    2: reacting to a stimulus; "the skin of old persons is less
       reactive than that of younger persons" [syn: {reactive},
       {responsive}]

from 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]

[email protected]