A random number added to a password to make it more difficult to crack. It is common practice to take passwords and run them through a hashing algorithm and store the results in the login database. When users enter their passwords, they are once again hashed and matched against the database. A salt is a random number added to the password prior to hashing to make the result more difficult to uncover by using a "brute force" dictionary attack.
Less widely used than a salt, a "pepper" is a fixed value added to the password. See
hash function,
dictionary attack and
password.