The Collaborative International Dictionary of English v.0.48:

Air \Air\ ([^a]r), n. [OE. air, eir, F. air, L. a["e]r, fr. Gr.
   'ah`r, air, mist, for 'a[digamma]hr, fr. root 'a[digamma] to
   blow, breathe, probably akin to E. wind. In sense 10 the
   French has taking a meaning fr. It. aria atmosphere, air, fr.
   the same Latin word; and in senses 11, 12, 13 the French
   meaning is either fr. L. aria, or due to confusion with F.
   aire, in an older sense of origin, descent. Cf. A["e]ry,
   Debonair, Malaria, Wind.]
   1. The fluid which we breathe, and which surrounds the earth;
      the atmosphere. It is invisible, inodorous, insipid,
      transparent, compressible, elastic, and ponderable.
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   Note: By the ancient philosophers, air was regarded as an
         element; but modern science has shown that it is
         essentially a mixture of oxygen and nitrogen, with a
         small amount of carbon dioxide, the average proportions
         being, by volume: oxygen, 20.96 per cent.; nitrogen,
         79.00 per cent.; carbon dioxide, 0.04 per cent. These
         proportions are subject to a very slight variability.
         Air also always contains some vapor of water.
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   2. Symbolically: Something unsubstantial, light, or volatile.
      "Charm ache with air." --Shak.
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            He was still all air and fire. [Air and fire being
      the finer and quicker elements as opposed to earth and
      water.]                                     --Macaulay
      .
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   3. A particular state of the atmosphere, as respects heat,
      cold, moisture, etc., or as affecting the sensations; as,
      a smoky air, a damp air, the morning air, etc.
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   4. Any a["e]riform body; a gas; as, oxygen was formerly
      called vital air. [Obs.]
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   5. Air in motion; a light breeze; a gentle wind.
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            Let vernal airs through trembling osiers play.
                                                  --Pope.
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   6. Odoriferous or contaminated air.
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   7. That which surrounds and influences.
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            The keen, the wholesome air of poverty.
                                                  --Wordsworth.
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   8. Utterance abroad; publicity; vent.
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            You gave it air before me.            --Dryden.
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   9. Intelligence; information. [Obs.] --Bacon.
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   10. (Mus.)
       (a) A musical idea, or motive, rhythmically developed in
           consecutive single tones, so as to form a symmetrical
           and balanced whole, which may be sung by a single
           voice to the stanzas of a hymn or song, or even to
           plain prose, or played upon an instrument; a melody;
           a tune; an aria.
       (b) In harmonized chorals, psalmody, part songs, etc.,
           the part which bears the tune or melody -- in modern
           harmony usually the upper part -- is sometimes called
           the air.
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   11. The peculiar look, appearance, and bearing of a person;
       mien; demeanor; as, the air of a youth; a heavy air; a
       lofty air. "His very air." --Shak.
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   12. Peculiar appearance; apparent character; semblance;
       manner; style.
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             It was communicated with the air of a secret.
                                                  --Pope.
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   12. pl. An artificial or affected manner; show of pride or
       vanity; haughtiness; as, it is said of a person, he puts
       on airs. --Thackeray.
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   14. (Paint.)
       (a) The representation or reproduction of the effect of
           the atmospheric medium through which every object in
           nature is viewed. --New Am. Cyc.
       (b) Carriage; attitude; action; movement; as, the head of
           that portrait has a good air. --Fairholt.
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   15. (Man.) The artificial motion or carriage of a horse.
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   Note: Air is much used adjectively or as the first part of a
         compound term. In most cases it might be written
         indifferently, as a separate limiting word, or as the
         first element of the compound term, with or without the
         hyphen; as, air bladder, air-bladder, or airbladder;
         air cell, air-cell, or aircell; air-pump, or airpump.
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   Air balloon. See Balloon.

   Air bath.
       (a) An apparatus for the application of air to the body.
       (b) An arrangement for drying substances in air of any
           desired temperature.

   Air castle. See Castle in the air, under Castle.

   Air compressor, a machine for compressing air to be used as
      a motive power.

   Air crossing, a passage for air in a mine.

   Air cushion, an air-tight cushion which can be inflated;
      also, a device for arresting motion without shock by
      confined air.

   Air fountain, a contrivance for producing a jet of water by
      the force of compressed air.

   Air furnace, a furnace which depends on a natural draft and
      not on blast.

   Air line, a straight line; a bee line. Hence

   Air-line, adj.; as, air-line road.

   Air lock (Hydr. Engin.), an intermediate chamber between
      the outer air and the compressed-air chamber of a
      pneumatic caisson. --Knight.

   Air port (Nav.), a scuttle or porthole in a ship to admit
      air.

   Air spring, a spring in which the elasticity of air is
      utilized.

   Air thermometer, a form of thermometer in which the
      contraction and expansion of air is made to measure
      changes of temperature.

   Air threads, gossamer.

   Air trap, a contrivance for shutting off foul air or gas
      from drains, sewers, etc.; a stench trap.

   Air trunk, a pipe or shaft for conducting foul or heated
      air from a room.

   Air valve, a valve to regulate the admission or egress of
      air; esp. a valve which opens inwardly in a steam boiler
      and allows air to enter.

   Air way, a passage for a current of air; as the air way of
      an air pump; an air way in a mine.

   In the air.
       (a) Prevalent without traceable origin or authority, as
           rumors.
       (b) Not in a fixed or stable position; unsettled.
       (c) (Mil.) Unsupported and liable to be turned or taken
           in flank; as, the army had its wing in the air.

   on the air, currently transmitting; live; -- used of radio
      and television broadcasts, to indicate that the images and
      sounds being picked up by cameras and microphones are
      being broadcast at the present moment.

   Note: In call-in programs where individuals outside a radio
         or television studio have telephoned into the station,
         when their voice is being directly broadcast, the host
         of the program commonly states "You're on the air." as
         a warning that the conversation is not private.

   To take air, to be divulged; to be made public.

   To take the air, to go abroad; to walk or ride out.
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WordNet (r) 3.0 (2006):

air compressor
    n 1: a compressor that takes in air at atmospheric pressure and
         delivers it at a higher pressure