The Collaborative International Dictionary of English v.0.48:

Ascension \As*cen"sion\, n. [F. ascension, L. ascensio, fr.
   ascendere. See Ascend.]
   1. The act of ascending; a rising; ascent.
      [1913 Webster]

   2. Specifically: The visible ascent of our Savior on the
      fortieth day after his resurrection. (--Acts i. 9.) Also,
      Ascension Day.
      [1913 Webster]

   3. An ascending or arising, as in distillation; also that
      which arises, as from distillation.
      [1913 Webster]

            Vaporous ascensions from the stomach. --Sir T.
                                                  Browne.
      [1913 Webster]

   Ascension Day, the Thursday but one before Whitsuntide, the
      day on which commemorated our Savior's ascension into
      heaven after his resurrection; -- called also Holy
      Thursday.

   Right ascension (Astron.), that degree of the equinoctial,
      counted from the beginning of Aries, which rises with a
      star, or other celestial body, in a right sphere; or the
      arc of the equator intercepted between the first point of
      Aries and that point of the equator that comes to the
      meridian with the star; -- expressed either in degrees or
      in time.

   Oblique ascension (Astron.), an arc of the equator,
      intercepted between the first point of Aries and that
      point of the equator which rises together with a star, in
      an oblique sphere; or the arc of the equator intercepted
      between the first point of Aries and that point of the
      equator that comes to the horizon with a star. It is
      little used in modern astronomy.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Ascension Day
    n 1: (Christianity) celebration of the Ascension of Christ into
         heaven; observed on the 40th day after Easter [syn:
         Ascension, Ascension Day, Ascension of the Lord]