AIX is the brand name of IBM's proprietary UNIX operating system. Several different versions of AIX have existed over time, some being eventually eliminated. The first version of AIX was released in 1986.
AIX is an acronym for Advanced Interactive eXecutive, it is almost always referred to by the acronym and hardly ever by the full name.
AIX had a reputation among its users for its maddening inconsistencies with other UNIX systems. A common joke about AIX is that it is an acronym standing for "Ain't unIX". (Compare GNU.) Others include "AIX and pains" and "AIX is UNIX for drunk aliens".
Clever ideas in AIX include commands like chuser, mkuser, rmuser and similar things which allows for user administration much the way you administrate files. AIX's level of support for volume management is gradually being added to the various free UNIX-like operating systems.