pseudonym

pseudonym [ SOOD-n-im ]
[ noun ]

MEANING :
1. a pen name
2. a fictitious name used by an author to conceal his or her identity
3. a nickname or alias

USAGE EXAMPLE 1 :
One of the employees wrote a letter to the manager criticizing the new policies being enforced by the company and signed it with a pseudonym.
USAGE EXAMPLE 2 :
Colonel Britton was the pseudonym of Douglas Ritchie, a 36-year-old Assistant News Editor at the BBC.