siglum
Concise Oxford English Dictionary © 2008 Oxford University Press:
siglum /ˈsɪgləm/
▶noun (pl. sigla /-lə/) a letter or symbol which stands for a word or name, especially to denote a particular manuscript or edition of a text.
– origin C18: from late L. sigla (pl.), perh. from singula, neut. pl. of singulus ‘single’.
'siglum' also found in these Oxford entries:

