What is the definition of "pseudonym"?

The term "pseudonym" specifically refers to a fictitious name used by an author to conceal their identity, which is precisely what is meant by an author's pen name. Writers often choose to use pseudonyms for various reasons, such as privacy, marketing strategies, or to write in different genres without preconceived notions attached to their real names.

While the other options touch upon names in different contexts, they do not capture the unique essence of a pseudonym. A formal title is an official designation, a nickname is often a casual or informal name given to someone (which may or may not relate to their real name), and a surname is the family name. None of these options align with the specific literary connotation and purpose encapsulated in the word "pseudonym."