What is your conceptual definition of ‘cell type’ in the context of a mature organism?

Hans Clevers, Susanne Rafelski, Michael Elowitz, Allon Klein, Jay Shendure, Cole Trapnell, Ed Lein, Emma Lundberg, Matthias Uhlen, Alfonso Martinez-Arias, Joshua R. Sanes, Paul Blainey, James Eberwine, Junhyong Kim and J. Christopher Love