Page "Orthography" Paragraph 6

Orthographic units, such as letters of an alphabet, are technically called graphemes.

These are a type of abstraction, analogous to the phonemes of spoken languages ; different physical forms of written symbols are considered to represent the same grapheme if the differences between them are not significant for meaning.

For example, different forms of the letter " b " are all considered to represent a single grapheme in the orthography of, say, English.