ah

from The Collaborative International Dictionary of English v.0.48

Ah \Ah\, interj. [OE. a: cf. OF. a, F. ah, L. ah, Gr. ?, Sk.
   [=a], Icel. [ae], OHG. [=a], Lith. ['a], ['a]['a].]
   An exclamation, expressive of surprise, pity, complaint,
   entreaty, contempt, threatening, delight, triumph, etc.,
   according to the manner of utterance.
   [1913 Webster]

from V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006)

AH
       [IP] Authentication Header (IPSEC, IPV6, RFC 1826, VPN)

from Easton's 1897 Bible Dictionary

Ah!
an exclamation of sorrow or regret (Ps. 35:25; Isa. 1:4, 24;
Jer. 1:6; 22:18; Mark 15:29).