The Collaborative International Dictionary of English v.0.48:

Nisus \Ni"sus\, n. [L., fr. niti, p. p. nisus, to strive.]
   1. A striving; an effort; a conatus.
      [1913 Webster]

            A nisus or energizing towards a presented object.
                                                  --Hickok.
      [1913 Webster]

   2. (Physiol.)
      (a) The periodic procreative desire manifested in the
          spring by birds, etc.
      (b) The contraction of the diaphragm and abdominal muscles
          to evacuate feces or urine.
          [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

nisus
    n 1: an effortful attempt to attain a goal [syn: striving,
         nisus, pains, strain]