In Postmodernist parlance, a simulacrum is a copy for which there is no original (or for which the original has no special value). A Taco Bell store might be a simulacrum: all the stores are basically copies in appearance and layout, but it's not like they're inferior copies of the Grand Original Taco Bell. If the original Taco Bell still exists, it doesn't have any special value above any of the copies.
Simulacra is just the Latin-nerd technically-correct plural form of simulacrum.
I mention the Postmodernist definition because it might be something fun to play with, if you're not doing a rules-heavy campaign (and if your player doesn't read this post). What if his character isn't a copy of a real mage, but rather is one of several almost-identical versions of a being for which there is no Original? Perhaps the five of them were created by an experimenting wizard.
When his simulacrum meets another simulacrum, they may both say, "I'm a copy of you!" Eventually, they'll realize that they're not a copy of any real being, but rather are all copies of one another. How they deal with this realization might be fun.
Daniel