Tenure is a modern coinage from the Latin, tenere, to hold.
Land was the major source of wealth, and the glue which held much of the social fabric together, both before and after the Conquest. Domesday Book is therefore dominated by landholding and its concepts. Tenure for service was the most significant of these and was particularly significant after the Conquest when most land had been re-distributed and its services recently defined or acknowledged. Apart from the Crown lands, all land after the Conquest was 'held from' the king or a tenant-in-chief, and granted in return for specified services. Such land was said to be subinfeudated. A subinfeudated manor could itself be further subinfeudated, creating a hierarchy of tenures.
Most land held in this way would have been held by a military tenure, the service of providing one or more men-at-arms when the king summoned the army, though Domesday Book does not normally record this fact. Whether this service was that previously incumbent upon Anglo-Saxon landowners, or whether the Conqueror had introduced a new form of service, the 'knight service' recorded in twelfth-century sources, remains a subject of debate among medievalists.
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); and Susan Reynolds, Fiefs and vassals: the medieval evidence reinterpreted (1994).