Generally the same as a password, or key, but used to indicate that it really should be composed of more than a single word. A passphrase is usually used as the source to derive the key used by the encryption algorithm. A fundamental problem with passphrases and passwords is that they need to be long to resist modern computer based attacks. In fact, to feel reasonable secure today, a passphrase should consist of at least 5 words, and they should be liberally modified by digits, other characters and in varying upper and lower case. Finally, a passphrase must of course not have any meaning as a sentence to be considered strong. As computer technology advances, it will be harder and harder for people memorize sufficiently long passphrases. My estimate is that the necessary length increases with one bit/year. Solutions to this problem is probably found by combining their use with a physical token, and possibly in some situations with biometrical solutions.