Sentence:  1° "The composition of two <WSD>morphisms</WSD> is defined if and only if the final object of the first <WSD>morphism</WSD> is the initial object of the second. This composition is also a <WSD>morphism</WSD>, whose initial object is the initial object of the first <WSD>morphism</WSD> and whose final object is the final object of the second." 
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.