Magitek Knights refers to a group of warriors in Final Fantasy VI. A Magitek knight is a soldier of the Gestahlian Empire that has been infused with Magitek and given the ability to cast magic, much like Terra Branford is able to do naturally due to her half-esper heritage. The exact process for creating Magitek knights is unknown, but it apparently involves some sort of augmentation to the brain, and according to Celes Chere, the subject is asleep during the procedure. Kefka Palazzo was the first experimental Magitek knight, but the process wasn't perfected yet and it had driven him insane. Later experiments refined the process to be more reliable. Kefka, however, was implied to be significantly stronger than Celes in terms of magical ability.
Both Celes and Terra eventually betray the Empire and join the Returners. Although Terra is considered an elite Magitek knight, she had natural magic not received from an infusion. General Leo Cristophe was also once considered to become a Magitek knight, but refused the infusion. Given the abilities of these three, it would not be inaccurate to surmise that Magitek knights, given their considerable sword skills and magical powers, would have been among the elite troops of the Empire. However, as Kefka shows, not all Magitek knights are as proficient in physical combat as they are in magic.
Terra, Celes and Kefka are the only named Magitek knights named in the game, as well as Leo, as he is a potential one. Emperor Gestahl himself also had access to magic, but he did not seem to possess any notable abilities in combat. Beyond this, Magitek knight seems to be a broad term for soldiers who can use magic, and thus, any soldier of the Empire who could cast magic could be considered one.