A code that prevents a wireless phone from unauthorized use. When explicitly locked, and each time the phone is turned on, it will prompt the user to enter the code before it will allow a call to be placed. Calls, such as emergency or other specially-programmed numbers, may be placed without entering a lock code.
Typical default codes include the last four digits of your phone number, the last four digits of your social security number, "1234", or "0000".
There are many other types of "locks", though:
See: Locked Phone
See: Master Subsidy Lock