a) alsothe Great Sphinxa large, very ancient sphinx which is close to the Pyramids of El Giza in Egypt and is visited by many tourists
b) in Greek mythology, a creature with the head of a woman and the body of a lion. She lay outside Thebes and killed people who could not answer her riddle (=very difficult question). Oedipus answered the riddle, and the Sphinx killed herself.
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.