A man-at-arms was a soldier from the High Medieval to Renaissance periods who was typically well-versed in the use of arms and served as a fully armoured heavy cavalryman. A man-at-arms could be a knight or nobleman, a member of a knight or nobleman's retinue or a mercenary in a company under a mercenary captain. Such men could serve for pay or through a feudal obligation. The terms knight and man-at-arms are often used interchangeably, but while all knights equipped for war certainly were men-at-arms, not all men-at-arms were knights.
In the Early Medieval period, any well-equipped horseman could be described as a "knight", or miles in Latin. In the course of the 12th century knighthood became a social rank with a distinction being made between milites gregarii (non-noble cavalrymen) and milites nobiles (true knights). As a fully armoured cavalryman could be of a lesser social status than a knight, an alternative term describing this type of soldier came into use which was, in French, homme d'armes or gent d'armes, and in English man-at-arms. The term man-at-arms thus primarily denoted a military function, rather than a social rank. This evolution differed in detail and timeline across Europe but by 1300, there was a clear distinction between the military function of the man-at-arms and the social rank of knighthood.
Though in English the term man-at-arms is a fairly straightforward rendering of the French homme d'armes, in the Middle Ages, there were numerous terms for this type of soldier. In France, he might be known as a lance or glaive, while in Germany a Spiess, Helm or Gleve and in various places a bacinet. In Italy, the term barbuta was used and in England from the late 14th century, men-at-arms were known as lances or its English equivalent, spears.
The military function that a man-at-arms performed was serving as a fully armoured heavy cavalryman; though he could, and in the 14th and 15th centuries often did, also fight on foot. In the course of the 16th century, the man-at-arms was gradually replaced by other cavalry types, the demi-lancer and the cuirassier, characterised by more restricted armour coverage and the use of weapons other than the heavy lance.