Domain Spells is a Cleric feature gained at level 3 in D&D 5e. Each domain has a list of spells--its domain spells--that you gain at the cleric levels noted in the domain description.
Each domain has a list of spells--its domain spells--that you gain at the cleric levels noted in the domain description. Once you gain a domain spell, you always have it prepared, and it doesn't count against the number of spells you can prepare each day.
If you have a domain spell that doesn't appear on the cleric spell list, the spell is nonetheless a cleric spell for you.