From Halopedia, the Halo wiki
Fleet Admiral, abbreviated to FADM, is a senior commissioned officer rank of O-10 in the United Nations Space Command Naval Command (NAVCOM) rank structure; including the UNSC Navy and Office of Naval Intelligence. It is preceded by Admiral, and is the highest naval rank.
Within the Covenant military, the title of Supreme Commander was considered analagous to the rank of Fleet Admiral - albeit with Supreme Commanders granted much greater increased independence than their UNSC counterparts.
Fleet Admiral - Halo 3 (Terrence Hood)