Sentence:  Approximately forty-five years ago K. Morita presented the first major results on equivalences and dualities between categories of <WSD>modules</WSD> over a pair of rings. 
Meaning of the word marked with <WSD> tags:  An abelian group equipped with the operation of multiplication by an element of a ring (or another of certain algebraic objects), representing a generalisation of the concept of "vector space" with "scalar multiplication".