The Collaborative International Dictionary of English v.0.48:

Militia \Mi*li"tia\, n. [L., military service, soldiery, fr.
   miles, militis, soldier: cf. F. milice.]
   [1913 Webster]
   1. In the widest sense, the whole military force of a nation,
      including both those engaged in military service as a
      business, and those competent and available for such
      service; specifically, the body of citizens enrolled for
      military instruction and discipline, but not subject to be
      called into actual service except in emergencies.
      [1913 Webster]

            The king's captains and soldiers fight his battles,
            and yet . . . the power of the militia is he. --Jer.
                                                  Taylor.
      [1913 Webster]

   2. Military service; warfare. [Obs.] --Baxter.
      [1913 Webster]

WordNet (r) 3.0 (2006):

militia
    n 1: civilians trained as soldiers but not part of the regular
         army [syn: militia, reserves]
    2: the entire body of physically fit civilians eligible by law
       for military service; "their troops were untrained militia";
       "Congress shall have power to provide for calling forth the
       militia"--United States Constitution

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

MILITIA. The military force of the nation, consisting of citizens called
forth to execute the laws of the Union, suppress insurrection and repel
invasion.
     2. The Constitution of the United States provides on this subject as
follows: Art. 1, s. 8, 14. Congress shall have power to provide for calling
forth the militia to execute the laws of the Union, suppress insurrections,
and repel invasions.
     3.-15. to provide for organizing, arming, and disciplining the
militia, and for governing such part of them as may be employed in the
service of the United States, reserving to the states respectively, the
appointment of the officers, and the authority of training the militia,
according to the discipline prescribed by congress.
     4. Under the clauses of the constitution, the following points have
been decided.
     1. If congress had chosen, they might by law, have considered a militia
man, called into the service of the United States, as being, from the time
of such call, constructively in that service, though not actually so,
although he should not appear at the place of rendezvous. But they have not
so considered him, in the acts of congress, till after his appearance at the
place of rendezvous: previous to that, a fine was to be paid for the
delinquency in not obeying the call, which fine was deemed an equivalent for
his services, and an atonement for disobedience.
     5.-2. The militia belong to the states respectively, and are subject,
both in their civil and military capacities, to the jurisdiction and laws of
the state, except so far as these laws are controlled by acts of congress,
constitutionally made.
     6.-3. It is presumable the framers of the constitution contemplated a
full exercise of all the powers of organizing, arming, and disciplining the
militia; nevertheless, if congress had declined to exercise them, it was
competent to the state governments respectively to do it. But congress has
executed these powers as fully as was thought right, and covered the whole
ground of their legislation by different laws, notwithstanding important
provisions may have been omitted, or those enacted might be beneficially
altered or enlarged.
     7.-4. After this, the states cannot enact or enforce laws on the same
subject. For although their laws may not be directly repugnant to those of
congress, yet congress, having exercised their will upon the subject, the
states cannot legislate upon it. If the law of the latter be the same, it is
inoperative: if they differ, they must, in the nature of things, oppose each
other, so far as they differ.
     8.-5. Thus if an act of congress imposes a fine, and a state law fine
and imprisonment for the same offence, though the latter is not repugnant,
inasmuch as it agrees with the act of the congress, so far as the latter
goes, and add another punishment, yet the wills of the two legislating
powers in relation to the subject are different, and cannot subsist
harmoniously together.
     9.-6. The same legislating power may impose cumulative punishments;
but not different legislating powers.
    10.-7. Therefore, where the state governments have, by the
constitution, a concurrent power with the national government, the former
cannot legislate on any subject on which congress has acted, although the
two laws are not in terms contradictory and repugnant to each other.
    11.-8. Where congress prescribed the punishment to be inflicted on a
militia man, detached and called forth, but refusing to march, and also
provided that courts martial for the trial of such delinquent's, to be
composed of militia officers only, should be held and conducted in the
manner pointed out by the rules and articles of war, and a state had passed
a law enacting the penalties on such delinquents which the act of congress
prescribed, and directing lists of the delinquents to be furnished to the
comptroller of the United States and marshal, that further proceeding might
take place according to the act of congress, and providing for their trial
by state courts martial, such state courts martial have jurisdiction.
Congress might have vested exclusive jurisdiction in courts martial to be
held according to their laws, but not having done so expressly, their
jurisdiction is not exclusive.
    12.-9. Although congress have exercised the whole power of calling out
the militia, yet they are not national militia, till employed in actual
service; and they are not employed in actual service, till they arrive at
the place of rendezvous. 5 Wheat. 1; Vide 1 Kent's Com. 262; 3 Story, Const.
Sec. 1194 to 1210.
    13. The acts of the national legislature which regulate the militia are
the following, namely: Act of May 8, 1792, 1 Story, L. U. S. 252; Act of
February 28, 1795, 1 Story, L. U. S. 390; Act of March 2, 1803, 2 Story, L.
U. S. 888; Act of April 10, 1806, Story, L. U. S. 1005; Act of April 20,
1816, 3 Story, L. U. S. 1573; Act of May 12, 1820, 3 Story, L. U. S. 1786
Act of March 2, 1821, 3 Story; L. U. S. 1811.