From Greek σύν (prep. "with"), πλησίος (adj. "close, near") and μορφή (noun “form”).
In cladistics, a symplesiomorphy is "the occurrence in two or more taxa of a monophyletic group of a plesiomorphic character or character state" (Kitching et al. 1998, 217).
References
– Kitching, Ian J., Peter L. Forey, Christopher J. Humphries, and David M. Williams. 1998. Cladistics: The Theory and Practice of Parsimony Analysis. 2nd ed. Oxford: Oxford University Press.
– Schmitt, Michael. 2013. From Taxonomy to Phylogenetics: Life and Work of Willi Hennig. Leiden: Brill.
In other languages
DE: symplesiomorph (adj.), Symplesiomorphie (noun)
FR: symplésiomorphique (adj.), symplésiomorphie (noun)
IT: simplesiomorfo (adj.), simplesiomorfismo (noun), simplesiomorfia (noun)