Definitions for "grapheme"
a written symbol that is used to represent speech.
minimally distinctive unit of writing in the context of a particular writing system. For example, and are distinct graphemes in English writing systems because there exist distinct words like big and dig. Conversely, a lowercase italiform letter and a lowercase Roman letter are not distinct graphemes because no word is distinguished on the basis of these two different forms. publishing & printing
Graphemes are the fundamental units in an orthography or writing system. The Spanish grapheme ch, for example, is treated as a single entity in sorting and often considered a separate letter. However, it is covered in Unicode by two characters, c + h, not a single ch character. (Anderson, 2003: 3)