Source : Webster's Revised Unabridged Dictionary (1913)

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

         A nisus or energizing towards a presented object.
                                                  --Hickok.

Nisus \Ni"sus\, n. (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.

Source : WordNet®

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