Admiral is one of the highest officer ranks in the Earth's forces, commonly used by officers with fleet command (flotilla or larger). In practice, "Admiral" can be used as a form of address for anyone bearing a rank containing that designation (Fleet Admiral, Rear Admiral, Vice Admiral, Flotilla Admiral, etc.); variations on the rank are not referenced in the series. Donald Hayes, Henry Gloval and Rick Hunter are all addressed and/or referenced as "Admiral."
See Ranks (Earth) for a complete overview of Earth forces ranks.