"that is, namely, to wit," late 14c., a Latin word used in English, "you may know, you may be sure, it is certain," used in sense "that is to say, namely," a contraction of scire licit "it is permitted to know," from scire "to know" (see science); for second element see licit. It was used as was Old English hit is to witanne, literally "it is to wit" (see wit (v.)). Often abbreviated sc. or scil.
Its function is to introduce : (a) a more intelligible or definite substitute, sometimes the English, for an expression already used ... (b) a word &c. that was omitted in the original as unnecessary, but is thought to require specifying for the present audience .... [Fowler]