American Heritage Dictionary

1. In mythology and legend, a man, often of divine ancestry, who is endowed with great courage and strength, celebrated for his bold exploits, and favored by the gods.
2. A person noted for feats of courage or nobility of purpose, especially one who has risked or sacrificed his or her life: soldiers and nurses who were heroes in an unpopular war.
3. A person noted for special achievement in a particular field: the heroes of medicine.  See Synonyms at celebrity.*
4. The principal male character in novel, poem, or dramatic presentation.  See Usage Note at heroine.**

* SYNONYMS:  celebrity, hero, luminary, name, notable, peronage.  The central meaning shared by these nouns is "a widely known person": social celebrities,; the heroes of science; a theatrical luminary; a big name in sports; a notable of the concert stage; a personage in the field of philosophy.

**The word hero should no longer be regarded as restricted to men in the sense "a person noted for courageous action," though heroine is always restricted to women.  The distinction between hero and heroine is still useful, however, in referring to the principal character of a fictional work, inasmuch as the virtues and qualities that become a traditional literary heroine like Elizabeth Bennet or Isabel Archer are generally quite different from those that become a traditional literary hero like Tom Sawyer or Huckleberry Finn.