The Collaborative International Dictionary of English v.0.48:

abortus \a*bort"us\ ([.a]*b[^o]rt"[u^]s), n.
   a fetus which has been aborted; same as abortion[2].
   [PJC]

WordNet (r) 3.0 (2006):

abortus
    n 1: a human fetus whose weight is less than 0.5 kilogram when
         removed or expelled from the mother's body

Bouvier's Law Dictionary, Revised 6th Ed (1856):

ABORTUS. The fruit of an abortion; the child born before its time, incapable
of life. See Abortion; Birth; Breath; Dead born; Gestation; Life.