The Collaborative International Dictionary of English v.0.48:

Expulsion \Ex*pul"sion\, n. [L. expulsio, fr. expellere: cf. F.
   expulsion. See Expel.]
   1. The act of expelling; a driving or forcing out; summary
      removal from membership, association, etc.
      [1913 Webster]

            The expulsion of the Tarquins.        --Shak.
      [1913 Webster]

   2. The state of being expelled or driven out.
      [1913 Webster]

WordNet (r) 3.0 (2006):

expulsion
    n 1: the act of forcing out someone or something; "the ejection
         of troublemakers by the police"; "the child's expulsion
         from school" [syn: ejection, exclusion, expulsion,
         riddance]
    2: squeezing out by applying pressure; "an unexpected extrusion
       of toothpaste from the bottom of the tube"; "the expulsion of
       pus from the pimple" [syn: extrusion, expulsion]
    3: the act of expelling or projecting or ejecting [syn:
       expulsion, projection, ejection, forcing out]

Moby Thesaurus II by Grady Ward, 1.0:

127 Moby Thesaurus words for "expulsion":
   banishment, blowout, booting out, cashiering, clearance,
   communication, conduction, contagion, convection, deconsecration,
   defenestration, defrocking, delivery, deportation, deposal,
   deposition, deprivation, detachment, dethronement, detrusion,
   diapedesis, diffusion, disbarment, disbarring, discard, discharge,
   discrownment, disenthronement, disgorgement, disjunction,
   dismissal, displacement, disposal, disposition, dissemination,
   ejaculation, ejection, ejectment, elimination, emission,
   eradication, eructation, eruption, eviction, excommunication,
   exile, expatriation, expelling, export, exportation, extradition,
   extravasation, extrusion, firing, forced resignation, impeachment,
   import, importation, interchange, jet, jettison,
   kicking downstairs, kicking upstairs, liquidation, metastasis,
   metathesis, metempsychosis, migration, mutual transfer, obtrusion,
   osmosis, ostracism, ouster, ousting, outburst, outlawing, outlawry,
   outpour, overthrow, overthrowal, passage, passing over,
   pensioning off, perfusion, purge, rejection, relegation, removal,
   repudiation, retirement, riddance, sacking, severance, spout,
   spread, spreading, spurt, squirt, superannuation, suspension,
   the boot, the bounce, the sack, throwing out, transduction,
   transfer, transfer of property, transference, transfusion, transit,
   transition, translation, translocation, transmigration,
   transmigration of souls, transmission, transmittal, transmittance,
   transplacement, transplantation, transposal, transposition, travel,
   unchurching, unfrocking, unseating, withdrawal



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

EXPULSION. The act of depriving a member of a body politic, corporate, or of
a society, of his right of membership therein, by the vote of such body or
society, for some violation of hi's. duties as such, or for some offence
which renders him unworthy of longer remaining a member of the same.
     2. By the Constitution of the United States, art. 1, s. 5, Sec. 2, each
house may determine the rules of its proceedings, punish its members for
disorderly behaviour, and, with the concurrence of two-thirds' expel a
member. In the case of John Smith, a senator from Ohio, who was expelled
from the senate in 1807, the committee made a report which embraces the
following points:
     3.-1. That the senate may expel a member for a high misdemeanor, such
as a conspiracy to commit treason. Its authority is not confined to an act
done in its presence.
     4.-2. That a previous conviction is, not requisite, in order to
authorize the senate to expel a member from their body, for a high: offence
against the United States.
     5.-3. That although a bill of indictment against a party for treason
and misdemeanor has been abandoned, because a previous indictment against
the principal party had terminated in an acquittal, owing to the
inadmissibility of the evidence upon that indictment, yet the senate may
examine the evidence for themselves, and if it be sufficient to satisfy
their. minds that the party is guilty of a high misdemeanor it is a
sufficient ground of expulsion.
     6.-4. That the 6th and 6th articles of the amendments of the
Constitution of the United States, containing the general rights and
privileges of the citizen, as to criminal prosecutions, refer only to
prosecutions at law, and do not affect the jurisdiction of the senate as to
expulsion.
     7.-5. That before a committee of the senate, appointed to report an
opinion relative to the honor and privileges of the senate, and the facts
respecting the conduct of the member implicated, such member is not entitled
to be heard in his defence by counsel, to have compulsory process for
witnesses, and to be confronted with his accusers. It is before the senate
that the member charged is entitled to be heard.
     8.-6. In determining on expulsion, the senate is not bound by the
forms of judicial proceedings, or the rules of judicial evidence; nor, it
seems, is the same degree of proof essential which is required to convict of
a crime. The power of expulsion must, in its nature, be discretionary, and
its exercise of a more summary character. 1 Hall's Law Journ. 459, 465.
     9. Corporations have the right of expulsion in certain cases, as such
power is necessary to the good order and government of corporate bodies; and
the cases in which the inherent power may be exercised are of three kinds.
1. When an offence is committed which has no immediate relation to a
member's corporate duty, but is of so infamous a nature as renders him unfit
for the, society of honest men; such as the offences of perjury, forgery,
and the like. But before an expulsion is made for a cause of this kind, it
is necessary that there should be a previous conviction by a jury, according
to the law of the land. 2. When the offence is against his duty as a
corporator, in which case he may be expelled on trial and conviction before
the corporation. 3. The third is of a mixed nature, against the member's
duty. as a corporator, and also indictable by the law of the land. 2
Binn.448. See, also, 2 Burr., 536.
    10. Members of what are called joint stock incorporated companies, or
indeed members of any corporation owning property, cannot, without express
authority in the charter, be expelled, and thus deprived of their interest
in the general fund. Ang. & Ames on Corp. 238. See; generally, Ang. & Ames
on Corp. ch. 11; Willcock, on Mun. Cor. 270; 1 Co. 99; 2 Bing. 293.; 5 Day
329; Sty. 478; 6 Conn. R. 532; 6 Serg. & Rawle, 469; 5 Binn. 486.