The PL/CS language is an instructional variant of PL/C designed to provide a simple, easy-to-understand tool to teach a disciplined style of programming (see [Conway 1976]). This report gives a complete formal semantic specification of the language, following the style of [Scott and Strachey 1972]. In keeping with the goal of simplicity in the design of PL/CS, the formal definition is presented in an hierarchical fashion and uses only elementary mathematical concepts, such as set, relation, and recursive definition. Key Words: programming language semantics, denotational semantics, recursive functions, Pl/I, PL/C, PL/CS.