The Collaborative International Dictionary of English v.0.48:

Aga \A*ga"\ or Agha \A*gha"\ ([.a]*g[aum]" or [=a]"g[.a]), n.
   [Turk. agh[=a] a great lord, chief master.]
   In Turkey, a commander or chief officer. It is used also as a
   title of respect.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Aga
    n 1: title for a civil or military leader (especially in Turkey)
         [syn: Aga, Agha]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

AGA
       Advanced Graphics Adapter