Medieval nobles serving as soldiers of higher rank and often of better training, usually depicted dressed in full platemail or lighter armor with decorative tunic over it.
Name variations: paladins
The first video game about Knights was released in 1975.
Often associated with the words chivalry, nobility, honor, and so forth.
Very similar in concept to Japanese samurai.
In fantasy settings you occasionally see Paladins who are either equivalent of knights or they are a mixture of knight and cleric (but maintaining the knightly appearance).