Formal:
* Pseudonym: Used for literary or artistic purposes.
* Nom de plume: French for "pen name," used by writers.
* Assumed name: A name taken on for a specific purpose, often to conceal identity.
* Fictitious name: A name that is not real.
* False name: A name used to deceive.
Informal:
* Nickname: A familiar or informal name.
* Handle: A name used online or in communication.
* Stage name: A name used by performers.
* Code name: A secret name used for identification.
* Screen name: A name used online, often for privacy.
* Fake name: A name used to mislead.
The best word to use depends on the specific situation and the intention behind the alias.