This paper considers the general problem of specifying the meaning of programming languages which include "data type definition facilities". The fundamental question posed in attempting to define such languages is: "what meaning should be given to a data type definition," or more simply, "what does data type mean?". In this paper we describe a new approach to defining the meaning of data types and give its application to the definition of a typed lambda calculus extension. We also prove a theorem stating that our language is "strongly typed".