A conjugate base is a substance that forms as the result of an acid losing a hydrogen ion. These bases are an essential part of the Brønsted acid-base theory, which states that all acid-base relations involve the transfer of a hydrogen ion or proton.
A conjugate base accepts the proton that the acid has lost. Bases can be weak or strong, depending on the acid. If an acid is strong, then its conjugate base is weak and vice versa. The stronger one of these items is, the weaker the other one is. An additional concept in the relation between acid and bases is that a conjugate base is always paired with a conjugate acid.
A conjugate base can be spotted within a chemical reaction by its negative charge. This charge is due to the loss of the hydrogen ion, which carries a positive charge. An example of this is nitric acid (HNO3) becoming the conjugate base nitrate (NO3).
Regardless of the strength or weakness of the conjugate base and other items within a reaction, water acts as a mitigating force that limits the strength of acids and bases. For example, water can pick up a hydrogen ion and become hydronium.