Pseudonym is an assumed name, a fictitious name used instead of the real name. Pseudonyms include pen-names, stage names, nicknames, aliases. Pseudonyms are often used by actors, singers, writers and other people for various reasons.
He uses a pseudonym to write short stories, but he writes novels under his real name.
This author writes under a pen-name.
He has several aliases. He travels under an alias of Smith.
a.k.a. — also known as: The police are looking for John Smith, a.k.a. Ben Little.