"That's it!” (3)
(Other definitions for aha that I've seen before include "Exclamation of triumph or mockery, a palindrome" , "I've got it!" , "I'm surprised" , "Expression of surprise, satisfaction or triumph" , "Cry of triumph" .)
(Other definitions for eureka that I've seen before include "Exultant cry" , "Archimedes's cry of discovery" , "Cry on a great discovery" , "Archimedes' cry of triumph as he ran naked through the streets of Syracuse?" , "Cry of triumph (at a discovery)" .)