I prefer the term "alternate universe." It just that this particular alternate universe is being termed the Mirror Universe by Kirk and later by the Federation.
IOW, the name's stuck.
Some could also theorize that perhaps this Mirror Universe is possibly the result of parallel realities, where a choice is made long ago about establishing a United Federation of Planets in the mutual interest of peaceful coexistance and common defense... or a conquering Terran Empire.
[This message has been edited by REG (edited 01-03-2001).]