passphrase (or pass phrase) is simply a password constructed of multiple words. Typically, though not always, the words in a passphrase are separated by spaces, as one would normally type.

The advantage of a passphrase is that it’s significantly easier to remember than complex passwords, and therefore can be significantly longer. Much longer passwords are generally considered more secure, even when they’re composed of common dictionary words. Even then, a simple alteration – perhaps replacing all of the spaces with periods, or all of the letter Os with number 0s, which can be easily remembered – can further obfuscate and make even dictionary-based attacks a practical impossibility.

The limiting factor is typically the system on which the password is to be used. Many do not accept spaces or allow for a sufficiently long password for a pass phrase to be used effectively.

One example of a passphrase is “correct horse battery staple“, made popular by the webcomic XKCD. It also shows that the phrase need not even make sense, as long as it’s memorable.

passphrase (Wikipedia)

A passphrase is a sequence of words or other text used to control access to a computer system, program or data. A passphrase is similar to a password in usage, but it is generally longer for added security. Passphrases are often used to control both access to, and the operation of, cryptographic programs and systems, especially those that derive an encryption key from a passphrase. The origin of the term is by analogy with password. The modern concept of passphrases is believed to have been invented by Sigmund N. Porter in 1982.

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