Neither. Lions are carnivores—specifically obligate (true) carnivores that rely on animal flesh for nutrition, not herbivores or omnivores.
National Geographic classifies the African lion (Panthera leo) with diet Carnivore. Pride females hunt large grassland prey such as antelope, zebra, and wildebeest; lions may also scavenge or take livestock near people, but plant matter is not a nutritional food source for them.