Merriam-Webster's Collegiate® Dictionary, 11th Edition
OPERATIVE
I. adjective
Date: 15th century
1. producing an appropriate effect ; efficacious , most significant or essential , exerting force or influence ; operating ,
3. having to do with physical operations (as of machines), working , based on or consisting of an operation , operatively adverb operativeness noun
II. noun
Date: circa 1810
operator : as, artisan , mechanic , a secret agent, private detective , a person who works toward achieving the objectives of a larger interest