It's different things, in different people's opinion. In the past, some have taken it to mean:

Imperial Rome (the first real nominee for the AntiChrist)

Emperor Nero

The Holy Roman Church

The Protestant Church

Martin Luther

The Pope

Stalin

Hitler

George W. Bush

the USA

Maitreya

and so on...and so on...just find out who a certain group of people are most prejudiced against and fearful of at any given time in history...and there is there (if not your) AntiChrist. :-)

It's a reflection of people's fear, is what it is.