Illustrated Glossary of Organic Chemistry

Base: (1) A Bronsted base or a Lewis base. In common chemistry usage, the term 'base' usually refers to a Bronsted base, whereas when a Lewis base is under discussion the terms 'Lewis base' or 'nucleophile' are used. Basicity refers to the base's ability to accept a proton. Hydroxide ion (HO^-) accepts a proton easily, and is strongly basic. Water does not accept a proton as readily as hydroxide ion does; water is weakly basic.

Hydroxide ion is a strong Bronsted base and good nucleophile. When hydroxide ion is under discussion as a proton acceptor (such as in its reaction with hydronium ion) it is called a base or a Bronsted base.
 

When hydroxide's role as an electron donor to an atom other than hydrogen is being considered (such as its role as a nucleophile in an S_N2 reaction), it is called a nucleophile or a Lewis base.