From 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]
From WordNet (r) 3.0 (2006) :
Aga
n 1: title for a civil or military leader (especially in Turkey)
[syn: Aga, Agha]
From V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016) :
AGA
Advanced Graphics Adapter