A traditional definition of cosplay, if such a thing even existed, would probably include some requirement to wear a costume. But you can sometimes achieve the same end result with nothing but make-up.
MadeYewLook is a pro when it comes to make-up, so much so that she can transform herself into characters like Alice, the Predator and even Rayman with just the work of a brush, some lighting and, in some cases, OK, something like a hat.
You can see more pics of MadeYewLook's make-up at her Facebook page, while there are tons of vids at her YouTube page (though be warned, seeing these things moving is sometimes as terrifying as it is impressive).
[thanks Fashionably Geek!]