The term thegn (or thane or thayn in Shakespearean English), from OE þegn, ðegn "servant, attendant, retainer", is commonly used to describe either an aristocratic retainer of a king or nobleman in Anglo-Saxon England, or as a class term, the majority of the aristocracy below the ranks of ealdormen and high-reeves. It is also the term for an early medieval Scandinavian class of retainers.

Old English þeg(e)n "servant, attendant, retainer" is cognate with Old High German degan and Old Norse þegn ("thane, franklin, freeman, man").

The thegn had a military significance, and its usual Latin translation was miles, meaning soldier, although minister was often used. Joseph Bosworth describes a thegn as "one engaged in a king's or a queen's service, whether in the household or in the country," and adds, "the word in this case seems gradually to acquire a technical meaning, and to become a term denoting a class, containing, however, several degrees."

But, like all other words of the kind, the word thegn was slowly changing its meaning, and, as William Stubbs says (Constitutional History, vol. i.), "the very name, like that of the gesith, has different senses in different ages and kingdoms, but the original idea of military service runs through all the meanings of thegn, as that of personal association is traceable in all the applications of gesith." After the Norman conquest of England in 1066, William the Conqueror replaced the Anglo-Saxon aristocracy with Normans and the new Norman French ruling class replaced the Anglo-Saxon terminology with Norman French. In this process, king's thegns became barons, and thegns appear to have been merged in the class of knights.