Lewis bases have available, unshared pairs of electrons. The definition of a Lewis base is more general the Brønsted-Lowry model of acid/base activity. When a Brønsted base receives a proton, it utilizes an electron pair to form the covalent bond, so its activity falls under the rubric of the Lewis system, but a Lewis acid is anything that might attach itself to such a pair of electrons, not only protons.

In organic chemistry, it is useful to understand that nucleophiles are Lewis bases and electrophiles are Lewis acids. Nearly all heterolytic reactions in organic chemistry can be thought of as Lewis acid-base processes.