It defines a proponent of meat-eating and human consumption of animals in general. It'd be what meat-eaters would call themselves if they could only accept that their diet stems from a philosophy of speciecism and a belief that humans are superior to other sentient beings.

Given that meat eating it is a choice and not an actual necessity in civilised societies, carnism is a philosophy that seeks to defend carnists in their abject subjugation and oppression of non-human animals by justifying their decision and making animals appear as mere objects.