Feudal relief was a one-off "fine" or form of taxation payable to an overlord by the heir of a feudal tenant to license him to take possession of his fief, i.e. an estate-in-land, by inheritance. It is comparable to a death duty or inheritance tax.
The equivalent duty at the lower levels of the feudal hierarchy was heriot (in England) or le droit du meilleur catel (in France).