The Collaborative International Dictionary of English v.0.48:

Pressure \Pres"sure\ (?; 138), n. [OF., fr. L. pressura, fr.
   premere. See 4th Press.]
   1. The act of pressing, or the condition of being pressed;
      compression; a squeezing; a crushing; as, a pressure of
      the hand.
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   2. A contrasting force or impulse of any kind; as, the
      pressure of poverty; the pressure of taxes; the pressure
      of motives on the mind; the pressure of civilization.
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            Where the pressure of danger was not felt.
                                                  --Macaulay.
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   3. Affliction; distress; grievance.
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            My people's pressures are grievous.   --Eikon
                                                  Basilike.
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            In the midst of his great troubles and pressures.
                                                  --Atterbury.
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   4. Urgency; as, the pressure of business.
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   5. Impression; stamp; character impressed.
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            All saws of books, all forms, all pressures past.
                                                  --Shak.
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   6. (Mech.) The action of a force against some obstacle or
      opposing force; a force in the nature of a thrust,
      distributed over a surface, often estimated with reference
      to the amount upon a unit's area.
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   7. Electro-motive force.
      [Webster 1913 Suppl.]

   Atmospheric pressure, Center of pressure, etc. See under
      Atmospheric, Center, etc.

   Back pressure (Steam engine), pressure which resists the
      motion of the piston, as the pressure of exhaust steam
      which does not find free outlet.

   Fluid pressure, pressure like that exerted by a fluid. It
      is a thrust which is normal and equally intense in all
      directions around a point. --Rankine.

   Pressure gauge, a gauge for indicating fluid pressure; a
      manometer.
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The Collaborative International Dictionary of English v.0.48:

Atmospheric \At`mos*pher"ic\, Atmospherical \At`mos*pher"ic*al\,
   a. [Cf. F. atmosph['e]rique.]
   1. Of or pertaining to the atmosphere; of the nature of, or
      resembling, the atmosphere; as, atmospheric air; the
      atmospheric envelope of the earth.
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   2. Existing or occurring in the atmosphere.
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            The lower atmospheric current.        --Darwin.
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   3. Caused, or operated on, by the atmosphere; as, an
      atmospheric effect; an atmospheric engine.
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   4. Dependent on the atmosphere. [R.]
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            In am so atmospherical a creature.    --Pope.
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   Atmospheric engine, a steam engine whose piston descends by
      the pressure of the atmosphere, when the steam which
      raised it is condensed within the cylinder. --Tomlinson.

   Atmospheric line (Steam Engin.), the equilibrium line of an
      indicator card. Steam is expanded "down to the atmosphere"
      when its pressure is equal to that of the atmosphere. (See
      Indicator card.)

   Atmospheric pressure, the pressure exerted by the
      atmosphere, not merely downwards, but in every direction.
      In amounts to about 14.7 Ibs. on each square inch.

   Atmospheric railway, one in which pneumatic power, obtained
      from compressed air or the creation of a vacuum, is the
      propelling force.

   Atmospheric tides. See under Tide.
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WordNet (r) 3.0 (2006):

atmospheric pressure
    n 1: the pressure exerted by the atmosphere [syn: atmospheric
         pressure, air pressure, pressure]