A temporal expression is a syntactic construct used in a query that evaluates to a temporal value, i.e., an event, an interval, a span, or a temporal element.
In snapshot databases, expressions evaluate to relations and therefore they may be called relational expressions to differentiate them from temporal expressions. All approaches to temporal databases allow relational expressions. Some only allow relational expressions, and thus they are unisorted. Some allow relational expressions, temporal expressions and also possibly boolean expressions. Such expressions may defined through mutual recursion.