dominus

Dominus, domini is second declension masculine, and it is built straight out of domus: the dominus is the man who holds the house. From there it covers the master of slaves, the owner of any property, and the head of a household, with domina as the feminine.

It is a good place to notice the second declension VOCATIVE, because you will meet it in the famous phrase before you meet it in a table. Nouns in -us make their vocative in -e, so addressing the man gives domine, not dominus and not domino. Domine, quo vadis is a vocative followed by a question.

Frequency warning. In Christian Latin Dominus is the standard rendering of "the Lord", used of God and of Christ, and it saturates the Vulgate, the liturgy and everything written under their influence. That single use is most of why the word sits at this rank; in Cicero it is common but ordinary. It gives dominate, dominion, domain, and in the modern languages Spanish don and dueno, Italian donna, French dame and English dame and madam.