A Hebrew word for God (see Hebrew words for God for some history of this and other words). It doesn't appear in the Bible, but is used as the pronunciation for YHWH, which does! Literally, it means "my lords", and is translated into English as "the Lord".
Note that Adonai is also a plural form, like Elohim. So it is possible that it was formed by some analogy to this earlier, biblical form.