Source : Free On-Line Dictionary of Computing

functional dependency
     
         Given a relation R (in a {relational database}),
        attribute Y of R is functionally dependent on attribute X of R
        and X of R functionally determines Y of R (in symbols R.X ->
        R.Y) if and only if each X in R has associated with it
        precisely one Y in R (at any one time).  Attributes X and Y
        may be {composite}.
     
        This is very close to a {function} in the mathematical sense.
     
        (1997-09-01)