Feudal relief
Feudal relief was a one-off "fine" or form of taxation payable to an overlord by the heir of a feudal tenant to licence 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).
Etymology
The word relief comes from the Latin verb levo, to raise, lift up, elevate, with the addition of the Latin inseparable adverbial particle re-, which has three distinct meanings: back, against and again. The Latin composite verb relevo results. The term used in mediaeval Latin charters is Relevius. The payment thus obtains an heir's "relief" by his being "re-elevated" or "lifted-up again" into the place of honour and privilege formerly occupied by his predecessor.
Rationale
As fiefs were originally granted by William the Conqueror as a reward for past service, there was no logical reason in feudal terms why the tenant's heir should take up the fief without himself having provided the king with some similar valuable past service. The payment of a fine for such "relief" can thus be seen within the logic of feudalism as payment in-lieu of such past service. Thus the right to inherit a fief was akin to a pre-emption right: the king would re-grant the fief to the heir in priority to a third party, so long as the heir paid his fine. This was a valuable right as the heir was more expert in the exploitation of the fief in question than of any other comparable estate of land, for example he was aware through experience of its optimal cropping rotation based on its differing soil-types and micro-climates, and he had available a local work-force loyal to his family.