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):

abortion
    n 1: termination of pregnancy
    2: failure of a plan [syn: miscarriage, abortion]

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

ABORTION, med jur. and criminal law. The expulsion of the foetus before the
seventh mouth of utero-gestation, or before it is viable. q.v.
     2. The causes of this accident are referable either to the mother, or
to the foetus and its dependencies. The causes in the mother may be: extreme
nervous susceptibility, great debility, plethora, faulty conformation, and
the like; and it is frequently induced immediately by intense mental
emotion.  The causes seated in the foetus are its death, rupture of the
membranes, &c.
     3. It most frequently occurs between the 8th and 12th weeks of
gestation.  When abortion is produced with a malicious design, it becomes a
misdemeanor, at common law, 1 Russell, 553; and the party causing it may be
indicted and punished.
     4. The criminal means resorted to for the purpose of destroying the
foetus, may be divided into general and local.  To the first belong
vivisection, emetics, cathartics diuretics, emmenagogues &c. The second
embraces all kinds of violence directly applied.
     5. When, in consequence of the means used to produce abortion, the
death of the woman ensues, the crime is murder.
     6. By statute a distinction is made between a woman quick with child,
(q.v.) and one who, though pregnant, is not so, 1 Bl. Com. 129.
Physiologists, perhaps with reason, think that the child is a living being
from the moment of conception. 1 Beck. Med. Jur. 291.
     General References. 1 Beck, 288 to 331; and 429 to 435; where will be
found an abstract of the laws of different countries, and some of the states
punishing criminal abortion; Roscoe, Cr. Ev. 190; 1 Russ. 553; Vilanova y
Manes, Materia Criminal Forense, Obs. 11, c. 7 n. 15-18.  See also 1 Briand,
Med. Leg. 1 ere partie, c. 4, where the question is considered, how far
abortion is justifiable, and is neither a crime nor a misdemeanor. See Alis.
Cr. L. of Scot. 628.