Pseudonyms are "false" names or names that are not the true (given) names of an individual. They are one of a larger group of -onyms and -nyms (like synonyms and antonyms) in English - many more than we were taught in school.
Pseudonyms, when used by an author, are called pen names.
Some Famous Pen Names
Richard Bachman is Stephen King, 20th century American horror author
Acton Bell, Currer Bell, and Ellis Bell were the names used by Anne Brontë, Charlotte Brontë, and Emily Brontë
Mary Westmacott is Agatha Christie, 20th century British mystery writer
Anthony Burgess is John Burgess Wilson, 20th century British writer, author of A Clockwork Orange.
Lewis Carroll is Charles Lutwidge Dodgson, 19th century British author, mathematician, known best for Alice's Adventures in Wonderland
Sue Denim is used by Dav Pilkey, a writer and illustrator of the popular "Captain Underpants" children's book series and is also used by science fiction writer Lewis Shiner. Sue Denim is a parody of the word pseudonym itself.
Isak Dinesen (Karen Blixen), 20th century Danish author of Out of Africa
H.D. (Hilda Doolittle), 20th century American poet, novelist and memoirist
George Eliot (Mary Ann Evans), 19th century English novelist
C. S. Forester (Cecil Smith), 20th century writer of the Captain Horatio Hornblower novels and The African Queen
O. Henry (William Sidney Porter), American author of short stories and novels
Hergé (Georges Remi), 20th century Belgian comics writer and artist, famous worldwide for creating the Tintin series of books
Ann Landers (Esther Pauline Friedman), and Abigail Van Buren/Dear Abby (Pauline Esther Friedman Phillips), advice columnists
Stan Lee (Stanley Martin Lieber), comic book pioneer & Spiderman creator
Molière (Jean Baptiste Poquelin), 17th century French theater writer, director and actor, and writer of comic satire.
George Orwell (Eric Arthur Blair), 20th century British author of Animal Farm and 1984
Ellery Queen Frederic Dannay and Manfred B. Lee shared this pen name for their 20th century detective fiction
Saki (Hector Hugh Munro), 20th century British short story writer and satirist
George Sand (Armandine Lucie Aurore Dupin), 19th century French novelist and early feminist
Dr. Seuss (Theodore Seuss Geisel), also used "Theo. LeSieg", 20th century American writer and cartoonist best known for his of children's books
Lemony Snicket is the listed author of A Series of Unfortunate Events but is really the pen name of Daniel Handler
Stendhal (Marie-Henri Beyle),19th century French writer
Mark Twain Samuel Langhorn Clemens (also used "Sieur Louis de Conte" for his fictional biography of Joan of Arc) 19th century American humorist, writer and lecturer
Voltaire (François-Marie Arouet), 18th century French Enlightenment writer, deist and philosopher