Glyphs and Allographs
In the same way that the surface forms of phonemes are speech sounds or phones (and different phones representing the same phoneme are called allophones), the surface forms of graphemes are glyphs (sometimes "graphs"), namely concrete written representations of symbols, and different glyphs representing the same grapheme are called allographs. Hence a grapheme can be regarded as an abstraction of a collection of glyphs that are all semantically equivalent.
For example, in written English (or other languages using the Latin alphabet), there are many different physical representations of the lowercase letter "a", such as a, ɑ, etc. But because the substitution of any of these for any other cannot change the meaning of a word, they are considered to be allographs of the same grapheme, which can be written ⟨a⟩.
Read more about this topic: Grapheme