The Collaborative International Dictionary of English v.0.48:

Indefinite \In*def"i*nite\, a. [L. indefinitus. See In- not,
   and Definite.]
   [1913 Webster]
   1. Not definite; not limited, defined, or specified; not
      explicit; not determined or fixed upon; not precise;
      uncertain; vague; confused; obscure; as, an indefinite
      time, plan, etc.
      [1913 Webster]

            It were to be wished that . . . men would leave off
            that indefinite way of vouching, "the chymists say
            this," or "the chymists affirm that." --Boyle.
      [1913 Webster]

            The time of this last is left indefinite. --Dryden.
      [1913 Webster]

   2. Having no determined or certain limits; large and
      unmeasured, though not infinite; unlimited; as, indefinite
      space; the indefinite extension of a straight line.
      [1913 Webster]

            Though it is not infinite, it may be indefinite;
            though it is not boundless in itself, it may be so
            to human comprehension.               --Spectator.
      [1913 Webster]

   3. Boundless; infinite. [R.]
      [1913 Webster]

            Indefinite and omnipresent God,
            Inhabiting eternity.                  --W. Thompson
                                                  (1745).
      [1913 Webster]

   4. (Bot.) Too numerous or variable to make a particular
      enumeration important; -- said of the parts of a flower,
      and the like. Also, indeterminate.
      [1913 Webster]

   Indefinite article (Gram.), the word a or an, used with
      nouns to denote any one of a common or general class.

   Indefinite inflorescence. (Bot.) See Indeterminate
      inflorescence, under Indeterminate.

   Indefinite proposition (Logic), a statement whose subject
      is a common term, with nothing to indicate distribution or
      nondistribution; as, Man is mortal.

   Indefinite term (Logic), a negative term; as, the not-good.

   Syn: Inexplicit; vague; uncertain; unsettled; indeterminate;
        loose; equivocal; inexact; approximate.
        [1913 Webster]

WordNet (r) 3.0 (2006):

indefinite article
    n 1: a determiner (as `a' or `some' in English) that indicates
         nonspecific reference