hapax legomenon
Concise Oxford English Dictionary © 2008 Oxford University Press:
hapax legomenon /ˌhapaks lɪˈgɒmɪnɒn/
▶noun (pl. hapax legomena /-mɪnə/) a term of which only one instance of use is recorded.
– origin C17: Gk, ‘a thing said once’, from hapax ‘once’ and the passive part. of legein ‘to say’.
'hapax legomenon' also found in these Oxford entries:

