Well, there were so many definitions, so I just copied and pasted from Dictionary.com..
1. to bring down to a smaller extent, size, amount, number, etc.: to reduce one's wei…ght by 10 pounds. 2. to lower in degree, intensity, etc.: to reduce the speed of a car. 3. to bring down to a lower rank, dignity, etc.: a sergeant reduced to a corporal 4. to treat analytically, as a complex idea. 5. to lower in price. 6. to bring to a certain state, condition, arrangement, etc.: to reduce glass to powder. 7. to bring under control or authority. 8. Cookery . to evaporate water from (a sauce, soup, or other liquid), usually by boiling. 9. Photography . to lessen the density of (an exposed negative). 10. to adjust or correct by making allowances, as an astronomical observation. 11. Mathematics . to change the denomination or form, but not the value, of (a fraction, polynomial, etc.). 12. Chemistry . a. to add electrons to. b. to deoxidize. c. to add hydrogen to. d. to change (a compound) so that the valence of the positive element is lower. 13. Chemistry, Metallurgy . to bring into the metallic state by separating from nonmetallic constituents. 14. to thin or dilute: to reduce paint with oil or turpentine. 15. to lower the alcoholic concentration of (spirits) by diluting with water. 16. Surgery . to restore to the normal place, relation, or condition, as a fractured bone. 17. Phonetics . to modify the quality of (a speech sound) to one of lesser distinctiveness, esp. to pronounce (an unstressed vowel) as (É) or another centralized vowel, as in the unstressed syllables of medicinal. -verb (used without object) 18. to become reduced. 19. to become lessened, esp. in weight. 20. to be turned into or made to equal something: All our difficulties reduce to financial problems. 21. Cell Biology . to undergo meiosis. ( Full Answer )