The term cnidaria is based on the root words meaning what?

The term Cnidaria is based on the root word "cnid-", which comes from the Greek word "κνίδη" (knidē), meaning "nettle".

This is because cnidarians, like jellyfish, anemones, and corals, are known for having stinging cells called cnidocytes which contain nematocysts, harpoon-like structures that inject venom into prey or predators.