NOUN (3)

1. an inscrutable person who keeps his thoughts and intentions secret;

2. (Greek mythology) a riddling winged monster with a woman's head and breast on a lion's body; daughter of Typhon;

3. one of a number of large stone statues with the body of a lion and the head of a man that were built by the ancient Egyptians;

The Collaborative International Dictionary of English v.0.48:

Sphinx \Sphinx\, n. [L., from Gr. sfi`gx, usually derived from
   sfi`ggein to bind tight or together, as if the Throttler.]
   1.
      (a) In Egyptian art, an image of granite or porphyry,
          having a human head, or the head of a ram or of a
          hawk, upon the wingless body of a lion.
          [1913 Webster]

                The awful ruins of the days of old . . .
                Or jasper tomb, or mutilated sphinx. --Shelley.
          [1913 Webster]
      (b) On Greek art and mythology, a she-monster, usually
          represented as having the winged body of a lion, and
          the face and breast of a young woman.

   Note: The most famous Grecian sphinx, that of Thebes in
         B[oe]otia, is said to have proposed a riddle to the
         Thebans, and killed those who were unable to guess it.
         The enigma was solved by [OE]dipus, whereupon the
         sphinx slew herself. "Subtle as sphinx." --Shak.
         [1913 Webster]

   2. Hence: A person of enigmatical character and purposes,
      especially in politics and diplomacy.
      [1913 Webster]

   3. (Zool.) Any one of numerous species of large moths of the
      family Sphingidae; -- called also hawk moth. See also
      tomato worm.
      [1913 Webster]

   Note: The larva is a stout naked caterpillar which, when at
         rest, often assumes a position suggesting the Egyptian
         sphinx, whence the name.
         [1913 Webster]

   4. (Zool.) The Guinea, or sphinx, baboon (Cynocephalus
      sphinx).
      [1913 Webster]

   Sphinx baboon (Zool.), a large West African baboon
      (Cynocephalus sphinx), often kept in menageries.

   Sphinx moth. (Zool.) Same as Sphinx, 3.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sphinx
    n 1: an inscrutable person who keeps his thoughts and intentions
         secret
    2: (Greek mythology) a riddling winged monster with a woman's
       head and breast on a lion's body; daughter of Typhon
    3: one of a number of large stone statues with the body of a
       lion and the head of a man that were built by the ancient
       Egyptians