The Collaborative International Dictionary of English v.0.48:

Altitude \Al"ti*tude\, n. [L. altitudo, fr. altus high. Cf.
   Altar, Haughty, Enhance.]
   1. Space extended upward; height; the perpendicular elevation
      of an object above its foundation, above the ground, or
      above a given level, or of one object above another; as,
      the altitude of a mountain, or of a bird above the top of
      a tree.
      [1913 Webster]

   2. (Astron.) The elevation of a point, or star, or other
      celestial object, above the horizon, measured by the arc
      of a vertical circle intercepted between such point and
      the horizon. It is either true or apparent; true when
      measured from the rational or real horizon, apparent when
      from the sensible or apparent horizon.
      [1913 Webster]

   3. (Geom.) The perpendicular distance from the base of a
      figure to the summit, or to the side parallel to the base;
      as, the altitude of a triangle, pyramid, parallelogram,
      frustum, etc.
      [1913 Webster]

   4. Height of degree; highest point or degree.
      [1913 Webster]

            He is [proud] even to the altitude of his virtue.
                                                  --Shak.
      [1913 Webster]

   5. Height of rank or excellence; superiority. --Swift.
      [1913 Webster]

   6. pl. Elevation of spirits; heroics; haughty airs. [Colloq.]
      --Richardson.
      [1913 Webster]

            The man of law began to get into his altitude. --Sir
                                                  W. Scott.
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   Meridian altitude, an arc of the meridian intercepted
      between the south point on the horizon and any point on
      the meridian. See Meridian, 3.
      [1913 Webster]

WordNet (r) 3.0 (2006):

altitude
    n 1: elevation especially above sea level or above the earth's
         surface; "the altitude gave her a headache" [syn:
         altitude, height]
    2: the perpendicular distance from the base of a geometric
       figure to the opposite vertex (or side if parallel)
    3: angular distance above the horizon (especially of a celestial
       object) [syn: elevation, EL, altitude, ALT]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "altitude":
   Cartesian coordinates, abscissa, apex, azimuth, ceiling, clearance,
   coordinates, cylindrical coordinates, declination, elevation,
   eminence, equator coordinates, exaltation, hauteur, height,
   heighth, highness, latitude, loftiness, longitude, ordinate, peak,
   perpendicular distance, polar coordinates, prominence,
   right ascension, stature, sublimity, summit, tallness,
   toploftiness