grapheme
Concise Oxford English Dictionary © 2008 Oxford University Press:
grapheme /ˈgrafiːm/
▶noun Linguistics the smallest meaningful contrastive unit in a writing system.
– derivatives
graphemic adjective,
graphemically adverb,
graphemics plural noun.
graphemic adjective,
graphemically adverb,
graphemics plural noun.
'grapheme' also found in these Oxford entries:

