Craig,

I'll save you some searching in the basement. C.J. Date (E.F. Codd's student) states the following in his book "Relational Database Selected Writings" Adison-Wesley ISBN: 0-201--14196-5 in the chapter 19 titled, "A Practical Approach to Database Desing", on page 430.

"Primary key (informal definition):

The primary key of a table is a field or field combination of that table that can be used as a unique identifier for the records of that table."

"Primary key (formal definition):

The primary key of a table is a set K of fields K1,K2,...,Kn of T(n>0) having the following two time-independent properties;

1, Uniqueness
At any given time, there cannot exist two disticnt records of T having the same value for K1, the same value for K2, ..., and the same value for Kn.

2, Minimality;
None of K1,K2,...,Kn can be discarded from K without destroying the uniqueness property."

Hope this helps.