Base
Definition
A substance that accepts hydrogen ions (protons): bitter taste, slippery feel, pH above 7.
Technical definition
A base is a proton (H+) acceptor in the Bronsted-Lowry model, or an electron-pair donor in the Lewis model. In water it increases the hydroxide concentration, giving pH above 7. Strong bases dissociate completely while weak bases have a Kb equilibrium.
Example
Ammonia (NH3) acts as a base when it accepts a proton to become NH4+. That is why ammonia cleaners feel slippery and smell sharp.
Common confusion
Bases do not have to contain OH-. Ammonia has no hydroxide yet is a classic base: basic describes proton-accepting behavior, not just hydroxide content.