Sentence:  In certain extreme cases in mathematics, the synthesis of <WSD>morphisms</WSD> and of transformations is so intimate that one can speak of a veritable fusion....Essentially, categories are sets of <WSD>morphisms</WSD> organized into operatory systems. 
Meaning of the word marked with <WSD> tags:  ("formally") An arrow in a category; ("less formally") an abstraction that generalises a map from one mathematical object to another and is structure-preserving in a way that depends on the branch of mathematics from which it arises.