in chemistry, any substance that in water solution is slippery to the touch, tastes bitter, changes the colour of indicators (e.g., turns red litmus paper blue), reacts with acids to form salts, and promotes certain chemical reactions (base catalysis). Examples of bases are the hydroxides of the alkali and alkaline earth metals (sodium, calcium, etc.) and the water solutions of ammonia or its organic derivatives (amines). Such substances produce hydroxide ions (OH^-) in water solutions (see Arrhenius theory).

Broader definitions of a base, to include substances that exhibit typical basic behaviour as pure compounds or when dissolved in solvents other than water, are given by the Brønsted-Lowry theory (Brønsted–Lowry theory) (q.v.) and the Lewis theory (q.v.). Compare acid.

in mathematics, an arbitrarily chosen whole number greater than 1 in terms of which any number can be expressed as a sum of that base raised to various powers. See numerals and numeral systems.