Source : Webster's Revised Unabridged Dictionary (1913)

Greyhound \Grey"hound`\, n.
   A swift steamer, esp. an ocean steamer.

Greyhound \Grey"hound`\, n. [OE. graihund, greihound, greahund,
   grihond, Icel. greyhundr; grey greyhound + hundr dog; cf. AS.
   gr[imac]ghund. The origin of the first syllable is unknown.]
   A slender, graceful breed of dogs, remarkable for keen sight
   and swiftness. It is one of the oldest varieties known, and
   is figured on the Egyptian monuments. [Written also
   {grayhound}.]

Source : WordNet®

greyhound
     n : a tall slender dog of an ancient breed noted for swiftness
         and keen sight; used as a racing dog