The name the people of Beta III gave to their society under Landru, in which every individual was absorbed into a placid communal will and outsiders were treated as an infection.
The Body is the whole population of Beta III as ruled by Landru, the machine that carried on the programme of a leader who had lived six thousand years earlier. Those who are of the Body are tranquil, obedient and without initiative, apart from the Festival, when the red hour releases a night of violence.
In The Return of the Archons, the landing party from the Enterprise is recognised at once as not of the Body. The robed Lawgivers try to have the visitors absorbed, and succeed for a time with McCoy and Sulu.
A small underground has kept its minds free and waits for outside help. Its members Reger and Marplon assist Kirk and Spock, who argue the computer into destroying itself by showing that it is harming the Body it exists to protect.