Dictionary: a difference that you can see, hear, smell, feel, etc. : a noticeable difference between things or people
: the separation of people or things into different groups
: importance, excellence, or achievement
Many people use the word wrongly... it doesn't really mean "difference", it is simply saying: this is THIS and NOT THAT. It shows that there is an invisible or visible else-ness between two things.
You are unique, because you are NOT anyone else... you are distinct, definable, but not separate.