Lewis & Short often use the abbreviation collat., standing for collateral according to the appendix of abbreviations. An example:
ŭtĕrus, i, m. (collat. form ŭter, Caecil. ap. Non. 188, 15; neutr. collat. form ŭtĕ-rum, i, Plaut. Aul. 4, 7, 10, acc. to Non. 229, 33; Turp. and Afran. ib.) [Sanscr. uttara, later; Gr. ὕστερος; cf. Gr. ὑστέρα, womb; Sanscr. udaram, belly; Engl. udder], the womb, matrix (syn. volva).
It is often used for the form of some word. I suspect it simply means "alternative". But the English word itself can also mean "descended from the same stock, but in a different line" in family relations. So what does it mean?