The Collaborative International Dictionary of English v.0.48:

Abortion \A*bor"tion\ ([.a]*b[^o]r"sh[u^]n), n. [L. abortio, fr.
   aboriri. See Abort.]
   1. The act of giving premature birth; particularly, the
      expulsion of the human fetus prematurely, or before it is
      capable of sustaining life; miscarriage.
      [1913 Webster]

   2. The immature product of an untimely birth; a fetus which
      has been delivered prematurely due to spontaneous or
      voluntary abortion, and is dead.
      [1913 Webster +PJC]

   3. (Biol.) Arrest of development of any organ, so that it
      remains an imperfect formation or is absorbed.
      [1913 Webster]

   4. Any fruit or produce that does not come to maturity, or
      anything which in its progress, before it is matured or
      perfect; a complete failure; as, his attempt proved an
      abortion.
      [1913 Webster]

   5. the removal of a fetus from the womb prior to normal
      delivery in a manner such as to cause the death of the
      fetus; also called voluntary abortion, or when performed
      by a physician, therapeutic abortion.
      [PJC]

   Note: In the 1913 Webster there was the following note
         appended to sense 1:
         [hand] It is sometimes used for the offense of
         procuring a premature delivery, but strictly the early
         delivery is the abortion, "causing or procuring
         abortion" is the full name of the offense. --Abbott.
         [PJC]

   6. something considered to be a repulsive or monstrous
      variant of a normal object; a monstrosity.
      [PJC]

WordNet (r) 3.0 (2006):

therapeutic abortion
    n 1: a legally induced abortion for medical reasons (as when the
         mother's life is threatened)