One of the names for God is Adonai, which is Hebrew for "Lord" (Hebrew: דֹנָי). Formally, this is a plural ("Lords"), but the plural is usually construed as a respectful, and not a syntactic plural. The singular form is Adoni ("lord"). This was used by the Phoenicians for the pagan god Tammuz and is the origin of the Greek name Adonis. Jews only use the singular to refer to a distinguished person.
Some suggest that "Adonai" and other names of God may be written in the plural form to point out that this one God embodies all of the many gods that were worshipped by the ancestors of the Israelites and concurrently by the surrounding peoples.
Since pronouncing God's personal name YHWH is considered sinful by the Jews, they use Adonai instead in prayers and the reading of the Scriptures. When the Masoretes added vowel pointings to the text of the Hebrew Bible in the first century A.D., they gave the word YHWH the vowels of Adonai, to remind the reader to say Adonai instead.