Symplesiomorphy is a term used in biology, although its use could be extended to other fields quite easily.
"Symplesiomorphy", while it is quite a mouthful, can also easily be easily taken apart into roots: "Sym-" ("Together") "Plesio" ("Early", as in the Plesiosaurus) and "morphy" ("Shape"). So a Symplesiomorphy is the "Together Earliest Shape", or to put it into more general syntax, "The earliest form that various creatures shared".
For example, the symplesiomorphy of mammals is that of a creature that was a proto-rodent. Later mammals developed forms as varied as tiny flying bats and gigantic swimming whales, but all of them share the symplesiomorphy, with some signs still evident, of a small, four legged terrestrial animal who lived by climbing and burrowing.
It is also suitable, although somewhat of a stretch, to speak of symplesiomorphy in technology. Although there have been many changes in aviation technology since the earliest days of aircraft, the symplesiomorphy of modern aircraft could be seen to be a single-engined monoplane with straight wings. Anything from a space shuttle to a stealth bomber to a Supersonic Transport could be seen as extensions of that.
I also use the concept of symplesiomorphy in culture. For example, one of the things that confused me as a Westerner when I visited Alabama was being classified as a Northerner. To me, people from the East Coast have an obvious cultural symplesiomorphy, whether they are from Arkansas or Maine, and the cosmetic differences their culture has developed are seen by me as extremely superficial features on the surface of that symplesiomorphy.