The Collaborative International Dictionary of English v.0.48:

Kaaba \Ka*a"ba\ (k[.a]*[=a]"b[.a]), prop. n. [Ar. ka'bah, lit.,
   a square building, fr. ka'b cube.]
   The small and nearly cubical stone building, in the court of
   the Great Mosque at Mecca, toward which all Mohammedans must
   pray. It contains a sacred black stone, believed by
   Mohammedans to be one of the precious stones of paradise, and
   to have been brought to Abraham when he was contructing the
   Kaaba, by the Angel Gabriel. The Kaaba itself predates
   Mohammed, having been a pantheon which contained Arab idols,
   which were destroyed by Mohammed. [Written also caaba,
   kaabeh and kaabah.]
   [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Caaba \Ca*a"ba\ (k[.a]*[=a]"b[.a]), n. [Ar. ka'bah, lit., a
   square building, fr. ka'b cube.]
   The small and nearly cubical stone building, toward which all
   Mohammedans must pray. [Written also kaaba.]
   [1913 Webster]

   Note: The Caaba is situated in Mecca, a city of Arabia, and
         contains a famous black stone said to have been brought
         from heaven. Before the time of Mohammed, the Caaba was
         an idolatrous temple, but it has since been the chief
         sanctuary and object of pilgrimage of the Mohammedan
         world.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Kaaba
    n 1: (Islam) a black stone building in Mecca that is shaped like
         a cube and that is the most sacred Muslim pilgrim shrine;
         believed to have been given by Gabriel to Abraham; Muslims
         turn in its direction when praying [syn: Kaaba, Caaba]