The Collaborative International Dictionary of English v.0.48:

Impeachment \Im*peach"ment\, n. [Cf. F. emp[^e]chement.]
   The act of impeaching, or the state of being impeached; as:
   (a) Hindrance; impediment; obstruction. [Obs.]
       [1913 Webster]

             Willing to march on to Calais,
             Without impeachment.                 --Shak.
   (b) A calling to account; arraignment; especially, of a
       public officer for maladministration.
       [1913 Webster]

             The consequence of Coriolanus' impeachment had like
             to have been fatal to their state.   --Swift.
   (c) A calling in question as to purity of motives, rectitude
       of conduct, credibility, etc.; accusation; reproach; as,
       an impeachment of motives. --Shak.
       [1913 Webster]

   Note: In England, it is the privilege or right of the House
         of Commons to impeach, and the right of the House of
         Lords to try and determine impeachments. In the United
         States, it is the right of the House of Representatives
         to impeach, and of the Senate to try and determine
         impeachments.
         [1913 Webster]

   Articles of impeachment. See under Article.

   Impeachment of waste (Law), restraint from, or
      accountability for, injury; also, a suit for damages for
      injury. --Abbott.
      [1913 Webster]

WordNet (r) 3.0 (2006):

impeachment
    n 1: a formal document charging a public official with
         misconduct in office

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

IMPEACHMENT, const. law, punishments. Under the constitution and laws of the
United States, an impeachment may be described to be a written accusation,
by the house of representatives of the United States, to the senate of the
United States, against an officer. The presentment, written accusation, is
called articles of impeachment.
     2. The constitution declares that the house of representatives shall
have the sole power of impeachment art. 1, s. 2, cl. 5 and that the senate
shall have the sole power to try all impeachments. Art. 1, s. 3, cl. 6.
     3. The persons liable to impeachment are the president, vice-president,
and all civil officers of the United States. Art. 2, s. 4. A question arose
upon an impeachment before the senate, in 1799, whether a senator was a
civil officer of the United States, within the purview of this section of
the constitution, and it was decided by the senate, by a vote of fourteen
against eleven, that he was not. Senate Journ., January 10th, 1799; Story on
Const. Sec. 791; Rawle on Const. 213, 214 Serg. Const. Law, 376.
     4. The offences for which a guilty officer may be impeached are,
treason, bribery, and other high crimes and misdemeanors. Art. 2, s. 4. The
constitution defines the crime of treason. Art. 3, s. 3. Recourse must be
had to the common law for a definition of bribery. Not having particularly
mentioned what is to be understood by "other high crimes and misdemeanors,"
resort, it is presumed, must be had to parliamentary practice, and the
common law, in order to ascertain what they are. Story, Sec. 795.
     5. The mode of proceeding, in the institution and trial of
impeachments, is as follows: When a person who may be legally impeached has
been guilty, or is supposed to have been guilty, of some malversation in
office, a resolution is generally brought forward by a member of the house
of representatives, either to accuse the party, or for a committee of
inquiry. If the committee report adversely to the party accused, they give a
statement of the charges, and recommend that he be impeached; when the
resolution is adopted by the house, a  committee is appointed to impeach the
party at the bar of the senate, and to state that the articles of
impeachment against him will be exhibited in due time, and made good before
the senate, and to demand that the senate take order for the appearance of
the party to answer to the impeachment. The house then agree upon the
articles of impeachment, and they are presented to the senate by a committee
appointed by the house to prosecute the impeachment; the senate then issues
process, summoning the party to appear at a given day before them, to answer
to the articles. The process is served by the sergeant-at-arms of the
senate, and a return is made of it to the senate, under oath. On the return-
day of the process, the senate resolves itself into a court of impeachment,
and the senators are sworn to do justice, according to the constitution and
laws. The person impeached is called to answer, and either appears or does
not appear. If he does not appear, his default is recorded, and the senate
may proceed ex parte. If he does appear, either by himself or attorney, the
parties are required to form an issue, and a time is then assigned for the
trial. The proceedings on the trial are conducted substantially as they are
upon common judicial trials. If any debates arise among the senators, they
are conducted in secret, and the final decision is given by yeas and nays;
but no person can be convicted without the concurrence of two-thirds of the
members present. Const. art. 1, s. 2, cl. 6.
     6. When the president is tried, the chief justice shall preside. The
judgment, in cases of impeachment shall not extend further than to removal
from office, and disqualification to hold and enjoy any office of honor,
trust, or profit under the United States. Proceedings on impeachments under
the state constitutions are somewhat similar. Vide Courts of the United
States.



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

IMPEACHMENT, evidence. An allegation, supported by proof, that a witness who
has been examined is unworthy of credit.
     2. Every witness is liable to be impeached as to his character for
truth; and, if his general character is good, he is presumed, at all times,
to be ready to support it. 3 Bouv. Inst. n. 3224, et seq.