A concept is an idea, a scheme, or a plan that has been thought of as a potential solution to some problem. Concept design, in the context of the mechanical design process, concerns the development and evaluation of concepts to solve the problems posed in the specification, or put more simply, how are you going to perform the move the designer has requested.
Valid stage machinery concepts will evolve in specificity from the beginning to the end of a project. Very early on in the process, a perfectly acceptable concept for a machine actuator might simply be the phrase “…use pneumatics!”, which would imply an air compressor, control valves, cylinders and associated hardware. Soon enough words alone will fail in easily ...