A disciple was a person who followed a religious or philosophical leader.
In 2268, James T. Kirk questioned the Platonians' considering themselves Plato's disciples, given that their cruel treatment of Alexander and the USS Enterprise landing party was inconsistent with the philosopher's teachings. (TOS: "Plato's Stepchildren")
Later that year, Kirk warned Lokai that if he died aboard the Enterprise, he wouldn't be able to inspire any more disciples. (TOS: "Let That Be Your Last Battlefield")
In Kirk's 2269 captain's log, he noted that Doctor Sevrin had assured Kirk that his disciples would obey the rules and regulations on the Enterprise. (TOS: "The Way to Eden")