Micaiah son of Imlah is a prophet in the Hebrew Bible. He gave a negative prophesy to Ahab on his request (Catholic Encyclopedia article). He is not the same as the titular prophet of the Book of Micah, also called "The Morasthite" to distinguish him from Micaiah, the son of Imlah. Wikipedia
The definitive Wikipedia entry for Micaiah. Wikipedia is the biggest multilingual free-content encyclopedia on the Internet. en.wikipedia.org/wiki/Micaiah