5 All that Scripture therefore, which is called the Old Testament, is handed down fourfold to them who desire to know it, according to history, according to ætiology, according to analogy, according to allegory. Do not think me silly for using Greek words. In the first place, because I have so received, nor do I dare to make known to you otherwise than I have received. Next you yourself perceive, that we have not in use terms for such things: and had I translated and made such, I should have been indeed more silly: but, were I to use circumlocution, I should be less free in treating: this only I pray you to believe, that in whatever way I err, I am not inflated or swollen in any thing that I do.
Thus (for example) it is handed down according to history, when there is taught what has been written, or what has been done; what not done, but only written as though it had been done. According to ætiology, when it is shown for what cause any thing has been done or said. According to analogy, when it is shown that the two Testaments, the Old and the New, are not contrary the one to the other. According to allegory, when it is taught that certain things which have been written are not to be taken in the letter, but are to be understood in a figure.
Source: On the Profit of Believing (New Advent)