The Oxford English Dictionary defines this unit of measurement as follows:
league lig, sb.1 Forms: 4-5 leghe, 4-6 lege, leuge, (4 lewge, 5 lewke, leuke, leeke), 5-6 leege, 6 legge, le(a)que, Sc. lig, 6-7 leag(e, 6- league. Late ME. leuge, lege, leghe, etc., ad. late L. leuga, leuca (= late Gr. leu'gh, leu'kh), according to Hesychius and Jordanes a Gaulish word; hence OFr. liue, liwe (mod.Fr. lieue), Pr. lega, legua, Cat. llegua, Sp. legua, Pg. legoa, Ital. lega.
a. An itinerary measure of distance, varying in different countries, but usually estimated roughly at about 3 miles; app. never in regular use in England, but often occurring in poetical or rhetorical statements of distance.
marine league: a unit of distance = 3 nautical miles or 3041 fathoms. Although the league appears never to have been an English measure, leuca occurs somewhat frequently in Anglo-Latin law-books (Bracton, Fleta, etc.); it is disputed whether in these works it means one mile or two.