Pre-shared key

In cryptography, a pre-shared key or PSK is a shared secret which was previously shared between the two parties using some secure channel before it is used. Such systems almost always use symmetric key cryptographic algorithms. 
The characteristics of this secret or key are determined by the system which uses it; some system require that such keys be in a specific format. It can be a password like 'bret13i', a passphrase like 'Idaho hung gear id gene', or a hexadecimal string like '65E4 E556 8622 EEE1'. The secret is used by all systems involved in the cryptographic processes used to secure the traffic between the systems. 
The term is used in Wi-Fi encryption such as WEP or WPA, where both the wireless access points (AP) and all clients "share" the same key. 
Since one weakpoint of the crypto system is the encryption algorithm's key, the key strength is important, and since the strength of a key is in part dependent on the key length, it is important to choose a key whose length is cryptographically secure (see password strength). There are several tools available to help one choose a strong key. The Diceware method is one example.