This list of known abbreviations has been accumulated by naval-history.net open_in_new editors.
The abbreviations have been used in the official log-books of Royal Navy ships from the World War 1 era, and/or in the log-books of United States vessels from the mid 19th century onwards.
If you want to see something corrected, or add items to the list, drop me a linemail.
(a) after rating
Acting
A.A.
Anti-aircraft
A.B.
Able Seaman
Abm.
Abeam
A.B.S.
Armed boarding steamer
a/c
Altered course
a/c
Account
A.C. Co.
Alaska Commercial Company
A.C.S.
Air Construction Service, Royal Engineers
A.D.
Artificer Diver
A.D.C.
Aide-de-camp
Adm.
Admiral
Adv.
Advance
A.E.
Artificer Engineer
A.F.O.
Admiralty Fleet Order
A.G., ArmGrd
Armed guard
Ah.
Ahead
A.H.Q.
Army headquarters
Aide
Aide of Survey (USC&GS)
A.M.C.
Armed merchant cruiser
A+N code
Army and Navy signal code
AoF
Admiral of the fleet
A.O.L.
Absent over leave
A.P., Ast. Pay.
Assistant Paymaster
A.P.
Armour piercing
A.P.M.
Assistant Provost Marshal
A.R.
Aiming rifle
A.S.
Apprentice Seaman
A.S.C.
Admiralty Sailing Craft (dinghy)
A.S.C.
Army Service Corps
A.S.G.
Acting Seaman Gunner
A.S. Office
Admiral Superintendent Office
Asst.
Assistant of Survey (USC&GS)
A.T.F
Auxiliary Ocean Tug, Fleet
A.T.N.
Apparent time at noon
A. to N.
Aid to Navigation
A.T.P.
Authorised (or allowed) to proceed
A.T.S.
Apparent time at ship
A.W.
Above water
AWOL
Absent over leave