The Collaborative International Dictionary
Reactiveness
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.
Wiktionary
reactiveness
n. The quality or state of being reactive.