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I may be a bit vague here, but I'm searching for a word to describe a particular concept: The abstract idea about how a grapheme is formed, i.e., the conception about what elements of a grapheme are essential to its identity. A direct graphical representation of this "idea" shows the grapheme in its most basic, neutral form. (This is something that may very well change over time, which frequently leads to the birth of allographs, or even new alphabets.)
An example: The modern grapheme "P" can be described as a vertical line, to which a closed semicircle is attached at the top, to the right. This is, I think you all agree, the natural, most neutral form. However, this was not always so: originally, the P is related to the Greek Π, with the difference that the Italic (pre-classical) Π had a shorter descender to the right (i.e. a form half way between Π and Γ. This was frequently rounded, but almost always with a large opening at the bottom:
Over time, this opening gradually became smaller and smaller:
And today, the normal form (ask any child) is with a closed semi-circle. The open form is still a perfectly valid glyph-variant, no question about that, but the closed form is what is regarded as the "essential" P.
Now, what is it that has changed? The P-grapheme's "morphology"? Its "topology"? Is there a standard term for this concept?