Cryptonym is a noun that refers to a secret name.

Our word of the day combines the prefix C-R-Y-P-T-O, meaning ‘secret’ with the Greek suffix O-N-Y-M which means ‘name.’ A cryptonym is something a spy might have or anyone else who may engage in secret behaviour.

I was embarrassed to be writing a biography of a teen singer, so I used a cryptonym. There was no way I’d put my own name in such a silly book.