this is called a key attribute!
There are different types of key attributes!
Primary key is the key that is actually chosen, for example a social security number.
There is also a unique key, say our finger prints! they also uniquely identify humans, but usually we prefer to use social security number when we list people (a unique key could be a primary key if we chose so)
We could also identify people by their social security number and a name, but this is unnecessary, as social security number already gives a full identification. Such a reduntant key is called super key.
