Magnate, from the Late Latin magnas, a great man, itself from Latin magnus 'great', designates a noble or other man in a high social position, by birth, wealth or other qualities. In reference to the Middle Ages, the term is often used to distinguish higher territorial landowners and warlords such as counts, earls, dukes, and territorial-princes from the baronage.
Magnates were a social class of wealthy and influential nobility in the Crown of the Kingdom of Poland and Grand Duchy of Lithuania (and later the Polish-Lithuanian Commonwealth), see Magnates of Poland and Lithuania
In Spain, since late Middle Ages there is the highest class of nobility who hold appellation of Grandee of Spain.
In Sweden, wealthiest medieval lords were known as storman (plural stormän), "great men", a similar description as magnate, and same meaning.
In England, the term magnate has often been applied to the extremely powerful nobles that Edward III created when he split his kingdom amongst his sons rather than choosing one son to inherit the entire kingdom. The ensuing conflict between these powerful nobles (and their successors) and whoever was king led to the aristocratic wars known as the Wars of the Roses. A similar class in the Gaelic world were the Flatha.
Magnates in England 
Eventually intermarriage between the royal family and the higher nobility led to large numbers of claimants to the throne, most of whom tried to attain the throne by usurping their predecessors. In this period, despite laws governing succession, most of the succession was determined not as much by primogeniture as by military victory.
In the Tudor period, after Henry VII defeated Richard III at Bosworth Field, Henry made a point of executing or neutralizing as many magnates as possible. Henry VII would make parliament attaint undesirable nobles and magnates, thereby stripping them of their wealth, protection from torture, and power. Henry VII also used the Court of the Star Chamber to have powerful nobles executed. Henry VIII continued this approach in his reign; he inherited a survivalistic mistrust of nobles from his father. Henry VIII ennobled very few men and the ones he did were all "new men": novi homines, greatly indebted to him and having very limited power.
See also 
- This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press.