Nom de plume is an English-language expression, a translation of pen name. The French term is nom de guerre, or "war name". Allonym is another synonym for pseudonym.
Some authors take on pen names to conceal their identity: for example the Brontes, who felt they would either not be published at all, or not taken seriously as women authors. Others to segregate different types of work: Lewis Carroll because as Charles Lutwidge Dodgson he wrote mathematics papers; Agatha Christie wrote romantic novels as Mary Westmacott. Pseudonyms are not always secret: Stendahl's real name was known: at least one critic disparaged his pen name as an affectation.
Pen names of famous authors include: