diazonium
Concise Oxford English Dictionary © 2008 Oxford University Press:
diazonium /ˌdʌɪəˈzəʊnɪəm/
▶noun [as modifier] Chemistry of or denoting an organic cation containing two nitrogen atoms bonded together, present in many synthetic dyes.
– origin C19: coined in Ger. from diazo- (indicating the presence of two nitrogen atoms) + the suffix -onium (from ammonium).
'diazonium' also found in these Oxford entries:

