Ammonium hydroxide is an alkali (base) formed when ammonia (NH3) is dissolved in water (H2O).
NH3 + H2O ↔ NH4OH
As seen above, the reaction is reversible. Ammonia itself is a base, dissolving it in water makes up this compound.
The formula for hydrogen hydroxide (which is commonly known as water) is H2O while the formula for nitrogen hydroxide (which is commonly known as nitric acid) is HNO3.
