Plesiomorphy meaning old form) is a primitive character of a group. 
A primitive character state for the taxa under consideration.
plesiomorphy An ancestral or primitive character. An evolutionary trait that is homologous within a particular group of organisms but is not unique to members of that group and therefore cannot be used as a diagnostic or defining character for the group. For example, vertebrae are found in zebras, cheetahs, and orangutans; but the common ancest...
a character state that is present in both outgroups and in the ancestors