The Collaborative International Dictionary of English v.0.48:

Protuberance \Pro*tu"ber*ance\, n. [Cf. F. protub['e]rance. See
   Protuberant.]
   That which is protuberant swelled or pushed beyond the
   surrounding or adjacent surface; a swelling or tumor on the
   body; a prominence; a bunch or knob; an elevation.
   [1913 Webster]

   Solar protuberances (Astron.), certain rose-colored masses
      on the limb of the sun which are seen to extend beyond the
      edge of the moon at the time of a solar eclipse. They may
      be discovered with the spectroscope on any clear day.
      Called also solar prominences. See Illust. in Append.
      [1913 Webster]

   Syn: Projection, Protuberance. protuberance differs from
        projection, being applied to parts that rise from the
        surface with a gradual ascent or small angle; whereas a
        projection may be at a right angle with the surface.
        [1913 Webster]

WordNet (r) 3.0 (2006):

protuberance
    n 1: something that bulges out or is protuberant or projects
         from its surroundings; "the gun in his pocket made an
         obvious bulge"; "the hump of a camel"; "he stood on the
         rocky prominence"; "the occipital protuberance was well
         developed"; "the bony excrescence between its horns" [syn:
         bulge, bump, hump, swelling, gibbosity,
         gibbousness, jut, prominence, protuberance,
         protrusion, extrusion, excrescence]
    2: the condition of being protuberant; the condition of bulging
       out; "the protuberance of his belly"