Another name used for God in the Old Testament is or Adonai. It is the plural form of Adon which means "Master" or "Lord." It conveys the idea of rulership or dominion.

Therefore, the Lord (Adonai), the LORD Almighty, will send a wasting disease upon his sturdy warriors; under his pomp a fire will be kindled like a blazing flame (Isa 10:16).

Isaiah also wrote.

See, the Lord (Adonai), the LORD Almighty, will lop off the boughs with great power. The lofty trees will be felled, the tall ones will be brought low (Isa 10:33).

When used in the Old Testament of God it is spelled with a capital "L" and then lower case letters (Lord). When used of humans all of the letters are lower case (lord).

The Term Is Applied to Humans

The term is not only applied to God, it is also used of human beings who master or lord over people.

So the servant placed his hand under the thigh of Abraham his master, and swore to him concerning this matter (Gen 24:9).

Sarah, the wife of Abraham, called him lord.

Sarah laughed to herself, saying, "After I have become old, shall I have pleasure, my lord being old also?"

Adonai Elohim

Adonai is often joined with Elohim indicating that God is the one who is Master and Lord and that we human beings are His subjects.

Adonai Yahweh

The term is also used with the divine name.

Abram said, "O Lord God, what will You give me, since I am childless, and the heir of my house is Eliezer of Damascus?" (Gen 15:2)

Summary

Adonai is a Hebrew word. It is the plural form of Adon means Lord or Master. It is a term used of both God and humans. It can also refer to false gods. When used in English versions in the Old Testament it is spelled Lord with a capital "L."