noun

1. the thing or part on which something rests; lowest part or bottom; foundation
2. the fundamental or main part, as of a plan, organization, system, theory, etc.
3. the principal or essential ingredient, or the one serving as a vehicle paint with an oil base
4. anything from which a start is made; basis
5. the point of attachment of a part of the body the base of the thumb
6. a center of operations or source of supply; headquarters, as of a military operation or exploring expedition
7. 1. the bottommost layer or coat, as of paint
   2. a makeup cream to give a desired color to the skin, esp. in the theater
8. Archit. the lower part, as of a column, pier, or wall, regarded as a separate unit
9. Baseball any of the four objects at the four corners of the infield that must be reached safely one after the other to score a run: three (first base, second base, and third base) are set above the ground while the fourth (home plate) is set flush with the ground
10. Chem.
    1. any compound that can react with an acid to form a salt, the hydroxyl of the base being replaced by a negative ion: in modern theory, any substance that produces a negative ion and donates electrons to an acid to form covalent bonds: in water solution a base tastes bitter, turns red litmus paper blue, and, in dissociation theory, produces free hydroxyl ions
    2. any of the two purines (adenine or guanine) or three pyrimidines (thymine, cytosine, or uracil) that are the key building blocks of nucleic acid
11. Dyeing a substance used for fixing colors
12. Electronics in some transistors, the region or layer of semiconductor material, acting as an electrode, that separates the emitter from the collector and receives an electric current of electrons or holes
13. Geom. the line or plane upon which a figure is thought of as resting the base of a triangle
14. Heraldry the lower portion of a shield
15. Linguis. any morpheme to which prefixes, suffixes, etc. are or can be added; stem or root
16. Math.
    1. a whole number, esp. 10 or 2, made the fundamental number, and raised to various powers to produce the major counting units, of a number system; radix
    2. any number raised to a power by an exponent
    3. a starting or reference figure or sum upon which certain calculations are made

Etymology: ME < OFr bas < L basis, basis

adjective

transitive verb based, basing bas′·ing

1. to make or form a base or foundation for
2. to put or rest (on) as a base or basis to base a guess on past experience
3. to place or station (in or at a base)