1. Chemistry. A substance that can combine with a proton (H+) and donate a pair of electrons to form a covalent bond. A base tastes bitterly in a aqueous solution and combines with acids to form salts. The pH is greater than 7. Opposed to acid.
2. Genetics. Component of the genetic material. See nucleic base.
Base in the sense of basis, foundation or ground plate refers to a supporting part or an underlying element upon which a structure is built.