Della Pittura (On Painting) by Italian humanist Leon Battista Alberti (1404-1472) is considered as one of the most important works in the history of Western art theory. In this book, Alberti constructed a system of terminologies and a set of principles, in order to structurally describe the content of paintings, to accurately convey the technique of paintings, and to systematically understand and evaluate the meaning and value of paintings. With the method of "quasimorphosis", this paper explores Alberti's artistic term "istoria" in three steps. Firstly, it traces the origin of the term back to the Classical Age; secondly, it describes the establishment of and changes to it in Medieval thought and scholarship, and finally, it discusses how Alberti defined a new category of art by using traditional term as an artistic principle.