A flashgun designed to clip to the top of the camera on its accessory shoe or to be used off-camera and fired remotely by cable, radio control or infra-red. External flashguns have more power than the camera’s built-in flash and a lot more flexibility in the way you can control and direct the light.
Wireless flash system used by Nikon to control one or more external Speedlights from one place. Speedlights can even be combined in ‘groups’ for more power or more sophisticated lighting effects.
A flash control mode on some Nikon DSLRs and external flashguns (Speedlights) which can fire other Speedlights remotely via infra-red. It’s possible to control quite complex lighting setups in this way, and it’s part of Nikon’s CLS (Creative Lighting System).