Algebraic theories in monoidal categories

We define a monoidal semantics for algebraic theories. The basis for the definition is provided by the analysis of the structural rules in the term calculus of algebraic languages. Models are described both explicitly, in a form that generalises the usual definition in sets; and from a category-theoretical point of view, as monoidal functors on suitable classifying categories. The semantics obtained includes as special cases both the semantics of ordinary algebraic theories in Cartesian categories, and the semantics of operads and multicategories over sets.