A string of words and characters that you type in to authenticate yourself. Passphrases differ from passwords only in length. Passwords are usually short - six to ten characters. Passphrases are usually much longer - up to 100 characters or more.
Modern passphrases were invented by Sigmund N. Porter in 1982.
Their greater length makes passphrases more secure.
Phil Zimmermann's popular encryption program PGP, for example, requires you to make up a passphrase that you then must enter whenever you sign or decrypt messages.