Answer:
The definition has changed through the centuries. Originally, a magician was a man who practiced sorcery in an effort to actually affect or alter the laws of nature.
In more modern times a magician is typically one who performs 'tricks' that have the appearance of altering the laws of nature. The effect, for example, of David Copperfield flying through the air with no visible means of support looks like magic. Or in a simpler form, pulling a quarter out from behind someone's ear can appear to be magical.
The ability and proficiency of the one performing will vary greatly. Thus if the secret is easily discovered then the illusion of magic is lost-- and with it the magic.