A character that is common to (virtually) all members of a group and that in itself, or in combination with other diagnostic characters, is unique to that group. For example: empodia pulviliform; branches of R crowded on costal margin; wing often with fine longitudinal wrinkles are all diagnostic characters for the taxon Stratiomyidae. Compare with differential character.
Tried to attach an image to a forum post. jpg, 32kB, 72dpi, no blanks, ... File name is correctly displayed, but when I click "Preview Thread" it just vanishes. Help!