A fictitious name, especially one used by an author.
Early 19th century from French pseudonyme, from Greek pseudōnymos, from pseudēs ‘false’ + onuma ‘name’.
Frank made a modest living selling barn burner romance novels using the pseudonym, Sue Downing. Nobody cared.
[This word is a substitute for the Lexico suggestion contronym which we have examined before. It made me think about the multitude of “-nym” words we have in English. I wonder how many there are?]
The floodgates are open. Let the deluge of answers begin.