Latin, herietum, relevatio.
In theory, heriot (herietum) and relief (relevatio) were distinct. The Anglo-Saxon heriot represented the 'return' on his death of arms (or stock) with which a lord was presumed to have endowed his man during his lifetime. By contrast, the Anglo-Norman relief was a payment by an heir for the right to succeed to an estate. The one was personal; the other tenurial; one was a death-duty; the other a succession tax.
In practice, however, heriots and reliefs were often confused, by contemporaries as well as historians. In this translation, the inherent ambiguity is reflected in the translation of 'relief' as 'death duty' in many cases.
For more detail, see Sir Fredrick Pollock and F.W. Maitland, The history of English law before the time of Edward I (second edition, 1898); A.L. Poole, Obligations of society in the XII and XIII centuries (1946); and Susan Reynolds, Fiefs and vassals: the medieval evidence reinterpreted (1994).