Brønsted-Lowry Acids and Bases:
Definition
Put at its very simple, the Brønsted-Lowry definition of an acid is a molecule that can provide a proton. The Brønsted-Lowry definition of a base is a molecule that can accept that proton.
An instance of a simple acid/base reaction is the reaction of ammonia along with water. In the below diagram, water loses a proton and is an acid. Ammonia accepts that proton and is the base.
Figure: Reaction of ammonia with water.