An alias is another name that a person uses, especially to hide their real name or to do something secretly.
Using an alias can be sneaky or mysterious.
Examples:
The spy used an alias to hide his real name.
She wrote the letter using her alias, not her real name.
The famous singer sometimes performs under an alias.
The cat's alias is 'Whiskers' because it has long, white whiskers.
The superhero's alias is 'Captain Courage' when he's fighting crime.
The detective knew the criminal's alias, but not his real name.