Heriot means 'war gear', a death duty paid by the higher nobility in Anglo-Saxon England. It took the form of the 'return' of the weapons and horses with which a lord was presumed to have endowed his man when the bond between them was forged. Heriots were paid right up to the Conquest, a strong indication of the continuing military ethos of the Anglo-Saxon nobility down to 1066. In the late Anglo-Saxon period, a tariff was enshrined in the laws of King Canute. According to this code, for example, the heriot of an earl was eight horses - four saddled and four unsaddled - four mail coats, four helmets, eight spears, eight shields, four swords, and a substantial payment in gold. In other words, the war gear for four fully-equipped warriors and their attendants. Heriots could be commuted to cash payments, and often were for the lesser nobility; Domesday Book records several examples.
As a death duty, heriots could be confused with reliefs, which strictly speaking were inheritance taxes rather than a death duties. After the Conquest, death duties and inheritance taxes became fused, and relief superseded heriots as far as the nobility was concerned.
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).