Concept: Authors sometimes write under a name different from their real name. Such a name is used to hide identity, create a literary identity, or publish under another name.
Step 1: Understand the given definition.
The question says that the author assumes a name to conceal or obscure identity.
\[
\text{Assumed name} \Rightarrow \text{Name different from real name}
\]
Step 2: Identify the correct term.
A pseudonym is a false or assumed name used by an author.
\[
\text{Pseudonym} = \text{Pen name}
\]
Step 3: Eliminate wrong options.
Fictious means imaginary or not real.
Illustrious means famous or respected.
Imaginary means existing only in imagination.
Therefore, the correct answer is Pseudonym.