What is the opposite of an Acid?

Bases are the chemical opposite of acids. Acids are defined as compounds that donate a hydrogen ion (H+) to another compound (called a base). Traditionally, an acid (from the Latin 'acidus' or 'acere' meaning sour) was any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion activity greater than in pure water, i.e. a pH less than 7.0. Correspondingly, a base was any compound that, when dissolved in water, gives a solution with a hydrogen ion activity lower than that of pure water, i.e. a pH higher than 7.0 at standard conditions.

A soluble base is also called an alkali. A reaction between an acid and a base is called neutralization and this neutralization results in production of water and a salt.