As the title of a work, or one of the divisions of a work: a lecture, treatise.
Chiefly in anglicizations of Latin titles..
Origin
Early 17th century (in an earlier sense). From post-classical Latin relection-, relectio action of reading again from classical Latin relect-, past participial stem of relegere to pick up again, to read over or out, to recount, in post-classical Latin also to read again + -iō. Compare lection, prelection.