In origin, the meaning of thane - also translated as thegn - was 'one who serves'; but by the late Anglo-Saxon period the word was often used as a synonym for a free man of almost any rank. Royal thanes were among the richest and most powerful men in the kingdom, and median - middling - thanes were prosperous landholders of local or even regional importance. Even lesser thanes were considered noble and their service was honourable: they paid taxes and performed military service. A lesser landowner who 'prospered so that he possessed fully five hides of his own' might attain this status.
But there were also very large numbers of thanes before the Conquest who held tiny holdings, so modest that they were almost indistinguishable from those of the more prosperous peasants, and might even be smaller. A thane was a free man, a noble man, but often one of very limited resources.
Anglo-Saxon thanes who survived the Conquest holding land from the king were normally grouped with the minor royal sergeants, in the collective fiefs found at the end of many counties. These can sometimes be shown to have developed into the sergeantries of the later medieval period. Even where no link can be demonstrated, the predominance of occupational bynames among these royal thanes of 1086 makes it likely that many later sergeantries had descended from their holdings and were therefore ultimately of English origin.
After the Conquest, the status of thanes became devalued and the term itself gradually dropped out of normal use. By the end of the eleventh century it had already become uncommon for thanes to be included among those groups addressed by royal writs.
For further information, see J.H. Round, The king's sergeants and officers of state with their coronation services (1911); Richard P. Abels, Lordship and military obligation in Anglo-Saxon England (1988); and Ann Williams, The English and the Norman Conquest (1995).