In mathematics, base refers to the number which is raised to a given power. This finds common use, for example, in the depiction of numbers, for instance, 10 is the base used in the decimal system, whereas 2 is the base in the binary system. Another mathematical meaning is described in the article on topology.
In chemistry, a base is the reactive complement to an acid. In Lewis acid-base theory, a base is a proton acceptor. A Lewis base, when dissociated in water, typically yields negatively-charged hydroxide ion and a complementary postitive ion.
The positive ion from a Lewis base can form a salt from the negative ion of a Lewis acid. For example, two moles of the base sodium hydroxide (NaOH) can combine with one mole of sulfuric acid (H2SO4) to form two moles of water and one mole of sodium sulfate.
2 NaOH + H2SO4 --> 2 H2O + Na2SO4
In the more general Brønsted-Lowry acid base theory, a base is defined as an electron-pair donor.
In baseball, a base is one of 3 bags placed at corners of the infield diamond that a player has to run to after hitting the ball.