Admiral

Admiral (大将, taishō) is a title of admiral signified senior command rank in the Marines. Admirals, also known generally as flag officers, were typically responsible for a high level military command at battle group or fleet level, and/or for administrative and political duties relating to Marine operations.

In One Piece, Admirals are shown as being the second highest rank behind the Fleet Admiral.

Marine Admiral
There are only three Marines with the title of Admiral in the Marines. They are known by their aliases, "Blue Pheasant" (Aokiji), "Red Dog" (Akainu), and "Yellow Monkey" (Kizaru). The Admirals and Fleet Admiral Sengoku are the only Marines within the World Government who are capable of issuing Buster Calls.

The Admirals are crowned the World Government's "Greatest Powers", and are the strongest individual fighters within the World Government and the Marines (excluding the Shichibukai).

Only one previous Admiral has been shown since One Piece began and that is Sengoku; this was seen during Episode 277.

Should a World Noble be opposed, or injured, an Admiral will be summoned to take down the assailants who did the damage.

Trivia

 * The two Admirals introduced so far (Kizaru and Aokiji) are both based upon Japanese actors and are drawn to resemble them. Both Admirals also have Logia class Devil Fruits (somehow related to the color in their aliases, blue for ice and yellow for light).
 * Their aliases are taken from Momotaro legends, where a Pheasant, a dog and a monkey assisted the boy hero on his adventures.