Admiral is the second-highest rank in the Marines organization, senior to a Vice Admiral and junior to the Fleet Admiral. There are only three admirals serving at a given time, and as the strongest Marine officers, they count among the most powerful characters within the One Piece world.
The admirals are crowned the World Government's "Greatest Military Powers" (最高戦力 Saikō Senryoku), being considered not only the strongest active Marine officers (aside from the fleet admiral) but the strongest individual fighters within the entire governmental structure. As such, the three men are respected and feared by the entire world.
The mere presence of just a single admiral is enough to cause wide-spread panic on an island and make the entire population flee from the island, including big name pirates with bounties over 100,000,000; the act of challenging all three of them at once is enough to bring great shock to witnesses.
Three Admirals of the Golden Age of Piracy
Living up to their statuses as the strongest individual fighters of the World Government, all three of the admirals that were active before the end of the Skyfall War possess tremendous levels of power in various fields of combat.
Being admirals, the three are capable of using Haki, and Advanced forms of Haki, and showed themselves to be extremely proficient when using advanced Busoshoku Haki together to block even the most powerful attack in the world. All three of them are also able to survive a strike imbued with Haki, despite it nullifying their abilities.
The difference in physical prowess between admirals and vice admirals also seems to be drastically high (with the exception of Daisuke, as powerful New World pirates are easily able to incapacitate a vice admiral, yet an Admiral is easily capable of over powering them.
Every Admiral is proficient in at least one fighting style, but they are usually capable of using two kinds.
Aside from having control over the thousands of soldiers who rank underneath them, the admirals are the only Marines, along with the fleet admiral, within the World Government who have the authority and ability to issue a Buster Call on any island they deem a threat. They are also the only ones who can grant other World Government agents permission to use the Buster Call. The admirals' only superiors, aside from the fleet admiral, are the Five Elders and the World Government Commander-in-Chief.
The admirals are the direct subordinates of the World Nobles. Should one be opposed or injured, an admiral can be summoned to take down the opposing party in question and/or the assailants who did the damage. Failure is not acceptable.
With sufficient evidence and reason, an admiral can revoke a Shichibukai of his/her title, while pardoning criminals for working under a Warlord. Although, in all fairness, this authority is also granted to lower ranking personnel with sufficient evidence.