The Collaborative International Dictionary of English v.0.48:

Usage \Us"age\, n. [F. usage, LL. usaticum. See Use.]
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   1. The act of using; mode of using or treating; treatment;
      conduct with respect to a person or a thing; as, good
      usage; ill usage; hard usage.
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            My brother
            Is prisoner to the bishop here, at whose hands
            He hath good usage and great liberty. --Shak.
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   2. Manners; conduct; behavior. [Obs.]
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            A gentle nymph was found,
            Hight Astery, excelling all the crew
            In courteous usage.                   --Spenser.
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   3. Long-continued practice; customary mode of procedure;
      custom; habitual use; method. --Chaucer.
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            It has now been, during many years, the grave and
            decorous
            usage of Parliaments to hear, in respectful silence,
            all expressions, acceptable or unacceptable, which
            are uttered from the throne.          --Macaulay.
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   4. Customary use or employment, as of a word or phrase in a
      particular sense or signification.
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   5. Experience. [Obs.]
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            In eld [old age] is both wisdom and usage.
                                                  --Chaucer.
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   Syn: Custom; use; habit.

   Usage: Usage, Custom. These words, as here compared,
          agree in expressing the idea of habitual practice; but
          a custom is not necessarily a usage. A custom may
          belong to many, or to a single individual. A usage
          properly belongs to the great body of a people. Hence,
          we speak of usage, not of custom, as the law of
          language. Again, a custom is merely that which has
          been often repeated, so as to have become, in a good
          degree, established. A usage must be both often
          repeated and of long standing. Hence, we speak of a
          "hew custom," but not of a "new usage." Thus, also,
          the "customs of society" is not so strong an
          expression as the "usages of society." "Custom, a
          greater power than nature, seldom fails to make them
          worship." --Locke. "Of things once received and
          confirmed by use, long usage is a law sufficient."
          --Hooker. In law, the words usage and custom are often
          used interchangeably, but the word custom also has a
          technical and restricted sense. See Custom, n., 3.
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WordNet (r) 3.0 (2006):

usage
    n 1: the act of using; "he warned against the use of narcotic
         drugs"; "skilled in the utilization of computers" [syn:
         use, usage, utilization, utilisation, employment,
         exercise]
    2: accepted or habitual practice [syn: custom, usage,
       usance]
    3: the customary manner in which a language (or a form of a
       language) is spoken or written; "English usage"; "a usage
       borrowed from French"

The Devil's Dictionary (1881-1906):

USAGE, n.  The First Person of the literary Trinity, the Second and
Third being Custom and Conventionality.  Imbued with a decent
reverence for this Holy Triad an industrious writer may hope to
produce books that will live as long as the fashion.