The Collaborative International Dictionary of English v.0.48:

Hourglass \Hour"glass`\, n.
   An instrument for measuring time, especially the interval of
   an hour. It consists of a glass vessel having two
   compartments, from the uppermost of which a quantity of sand,
   water, or mercury occupies an hour in running through a small
   aperture unto the lower.
   [1913 Webster]

   Note: A similar instrument measuring any other interval of
         time takes its name from the interval measured; as, a
         half-hour glass, a half-minute glass. A
         three-minute glass is sometimes called an egg-glass,
         from being used to time the boiling of eggs.
         [1913 Webster]

WordNet (r) 3.0 (2006):

hourglass
    n 1: a sandglass that runs for sixty minutes

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "hourglass":
   abbreviation, astriction, astringency, bottleneck, cervix,
   circumscription, coarctation, compactedness, compaction,
   compression, compressure, concentration, condensation,
   consolidation, constriction, constringency, contraction,
   contracture, curtailment, decrease, diminuendo, hourglass figure,
   isthmus, knitting, narrow place, narrowing, neck, puckering,
   pursing, reduction, shortening, solidification, stranglement,
   strangulation, striction, stricture, systole, wasp waist,
   wrinkling