The Collaborative International Dictionary of English v.0.48:

Felony \Fel"o*ny\, n.; pl. Felonies. [OE. felonie cruelty, OF.
   felonie, F. f['e]lonie treachery, malice. See Felon, n.]
   1. (Feudal Law) An act on the part of the vassal which cost
      him his fee by forfeiture. --Burrill.
      [1913 Webster]

   2. (O.Eng.Law) An offense which occasions a total forfeiture
      either lands or goods, or both, at the common law, and to
      which capital or other punishment may be added, according
      to the degree of guilt.
      [1913 Webster]

   3. A heinous crime; especially, a crime punishable by death
      or imprisonment.
      [1913 Webster]

   Note: Forfeiture for crime having been generally abolished in
         the United States, the term felony, in American law,
         has lost this point of distinction; and its meaning,
         where not fixed by statute, is somewhat vague and
         undefined; generally, however, it is used to denote an
         offense of a high grade, punishable either capitally or
         by a term of imprisonment. In Massachusetts, by
         statute, any crime punishable by death or imprisonment
         in the state prison, and no other, is a felony; so in
         New York. the tendency now is to obliterate the
         distinction between felonies and misdemeanors; and this
         has been done partially in England, and completely in
         some of the States of the Union. The distinction is
         purely arbitrary, and its entire abolition is only a
         question of time.
         [1913 Webster]

   Note: There is no lawyer who would undertake to tell what a
         felony is, otherwise than by enumerating the various
         kinds of offenses which are so called. originally, the
         word felony had a meaning: it denoted all offenses the
         penalty of which included forfeiture of goods; but
         subsequent acts of Parliament have declared various
         offenses to be felonies, without enjoining that
         penalty, and have taken away the penalty from others,
         which continue, nevertheless, to be called felonies,
         insomuch that the acts so called have now no property
         whatever in common, save that of being unlawful and
         purnishable. --J. S. Mill.
         [1913 Webster]

   To compound a felony. See under Compound, v. t.
      [1913 Webster]

WordNet (r) 3.0 (2006):

felony
    n 1: a serious crime (such as murder or arson)

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "felony":
   atrocity, breach, crime, crime against humanity, deadly sin,
   delict, delinquency, dereliction, enormity, error, evil, failure,
   fault, genocide, guilty act, heavy sin, illegality, impropriety,
   indiscretion, inexpiable sin, iniquity, injury, injustice, lapse,
   malefaction, malfeasance, malum, minor wrong, misdeed, misdemeanor,
   misfeasance, mortal sin, nonfeasance, offense, omission, outrage,
   peccadillo, peccancy, sin, sin of commission, sin of omission,
   sinful act, slip, tort, transgression, trespass, trip,
   unutterable sin, venial sin, violation, wrong, wrongdoing



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

FELONY, crimes. An offence which occasions a total forfeiture of. either
lands or goods, or both, at common law, to which capital or other punishment
may be super-added, according to the degree of guilt. 4 Bl. Com, 94, 5; 1
Russ. Cr. *42; 1 Chit. Pract. 14; Co. Litt. 391; 1 Hawk. P. C. c. 37; 5
Wheat. R. 153, 159.