Nitrate, or NO3-, is the conjugate base of HNO3. The reaction resulting in the conjugate base of HNO3 is HNO3 + H2O → H3O+ + NO3-.
In most cases, the acid molecule that remains after losing a hydrogen ion is an acid's conjugate base. In this example, that conjugate base is the nitrate. To conjugate an acid, it is necessary to place it in water to create a chemical reaction with the H2O molecules. HNO3 is an acid, and it releases a proton into the water as a result of the chemical reaction. This results in the creation of the nitrate conjugate base and a hydronium (H3O+) acid base.