There are two modern definitions of a base in chemistry. These are as follows:


1. Brønsted base: A chemical species that combines with hydrogen ions (H+) – i.e., can bind with protons – when dissolved in water, thereby lowering the hydrogen ion concentration in the solution. Basic (or alkaline) solutions have pH values above 7.


2. Lewis base: A chemical species that has a lone pair of electrons available for coordinate bonding.


A base is the complement of an acid. In fact, bases used to be defined as substances that react with acids to give salts, or as substances that give rise to hydroxyl ions in aqueous solution. Some such inorganic strong bases are known as alkalis.