\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.

\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.