The Collaborative International Dictionary of English v.0.48:

Exertion \Ex*er"tion\, n.
   The act of exerting, or putting into motion or action; the
   active exercise of any power or faculty; an effort, esp. a
   laborious or perceptible effort; as, an exertion of strength
   or power; an exertion of the limbs or of the mind; it is an
   exertion for him to move, to-day.

   Syn: Attempt; endeavor; effort; essay; trial. See Attempt.
        [1913 Webster]

WordNet (r) 3.0 (2006):

exertion
    n 1: use of physical or mental energy; hard work; "he got an A
         for effort"; "they managed only with great exertion" [syn:
         effort, elbow grease, exertion, travail, sweat]