אָדוֹן (adon) and אֲדֹנָי (adonay)

The Hebrew word אָדוֹן (adon, “master, lord”) is not specifically a divine title. It can be used of humans, indicating a person who has authority (e.g., Judg 19:11; Gen 45:8). It is sometimes used to describe God, emphasizing His authority (e.g., Josh 3:13).

The divine title אֲדֹנָי (adonay) is related to אָדוֹן (adon) and is used only of God (e.g., Psa 2:4). Its form is very close to and may be derived from אֲדֹנַי (adonay, “my lords”), which is simply the plural of אָדוֹן (adon) followed by a first-person singular suffix (e.g., Gen 19:2); but it has a short a vowel in the suffix while the divine title אֲדֹנָי (adonay) has a long a vowel. The divine title may have originated as a respectful title used to address God (e.g., Exod 4:10), using a plural form to express extra respect. However, in the Old Testament it is used not merely to address God, but also to talk about Him (e.g., 2 Kgs 7:6).

For further details, see this article: Adonai.