WordNet (r) 3.0 (2006):

functioning
    adj 1: performing or able to perform its regular function; "a
           functioning flashlight" [ant: malfunctioning,
           nonfunctional]
    n 1: process or manner of functioning or operating; "the power
         of its engine determines its operation"; "the plane's
         operation in high winds"; "they compared the cooking
         performance of each oven"; "the jet's performance conformed
         to high standards" [syn: operation, functioning,
         performance]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "functioning":
   act, acting, action, active, activism, activity, agency, alive,
   at work, behavior, behavioral, conduct, direction, doing, driving,
   dynamic, employment, execution, exercise, function, functional,
   going, going on, handling, in exercise, in force, in hand,
   in operation, in play, in practice, in process, in the works,
   inaction, live, management, manipulation, movements, occupation,
   on foot, on the fire, ongoing, operancy, operating, operation,
   operational, operations, operative, performance, performing, play,
   practice, practicing, praxis, responsibility, running, serving,
   steering, swing, work, working, workings