Servitium liberum
Black's Law Dictionary · Henry Campbell Black, M.A. · 1910
Black's Law Dictionary
A service to be done by feudatory tenants, who were called "Uberi homines," and distinguished from vassals, as was their service, for they were not bound to any of the base services of plowing the lord's land, etc., but were to find a man and horse, or go with the lord into the army, or to attend the court, etc.
Cowell.