Collard greens are most famously used in the dish simply called ‘collard greens’ or ‘collards’. The name ‘collards’ however is derived from the same word as other brassicas its closely related to. From On Food and Cooking by Harold McGee;

Several vegetables in the cabbage family, ‘kale’, ‘collards’, and ‘cauliflower’, have names that derive from the Latin word ‘caulis,’ meaning “stem” or “stalk,” the part of the plant from which the edible portions emerge. ‘Cabbage’ itself comes from the Latin ‘caput,’ meaning “head.”