I've recently come across rocketlab's upper stage called kickstage which is the final deploying stage for their dedicated launch service. Is this just a fancier name for an upper stage? Have there been any previous concepts of kickstages for the upper stage designs? Is this considered a stage?
"Kick stage" or "apogee kick motor" is a loosely defined term for the last stage of a satellite launcher. Frequently it's the stage that puts the satellite into its final orbit. The term is not specific to RocketLabs.
It might be developed independently of the rest of the launcher; for example a given launcher might be able to launch large payloads directly into low Earth orbit, or put smaller payloads into a geosynchronous transfer orbit (GTO) (that is, an elliptical trajectory going from LEO to geosynchronous altitude), and a "kick stage" attached to the payload could put it into the final geosynchronous orbit.