The reasons that make a thing defensible. Also the even alignment of text to both edges.
Latin iustificare (to do justice to, make righteous), from iustus (just) + facere (to make); the reasons that make a thing defensible, and, in typesetting, the even alignment that makes a column look right.
The reasons that let a thing stand up to being questioned, and, in a different register, the even setting of text against both edges — defence and alignment named by the same word.