Temporal functional dependencies are generalizations of conventional functional dependencies. In the definition of a temporal functional dependency, a temporal relation is perceived as a collection of snapshot relations. Each such snapshot of any extension must satisfy the corresponding functional dependency.
Other (conflicting) notions of of temporal dependencies and keys have been defined, but none are as closely paralleled by snapshot dependencies and keys as the above. The naming of the concepts is orthogonal with respect to existing snapshot concepts, and the new names are mutually consistent (+E1, +E7).
Related notions of independent and dependent attributes exist. Using temporal as a prefix distinguishes the concept from conventional dependencies and points to the specific nature of the dependency. Thus ambiguity is avoided (+E5), and precision is enhanced (+E9)—at the expense of brevity (-E2).
``Temporal dependency'' has also been used in a non-generic sense, to denote a different concept. The term ``temporal'' is often used in a generic sense, so ambiguity results when it is also used in a specific sense. Thus ``temporal'' is used here only in a generic sense.