What is Call Forwarding?

Legal Definition
Call forwarding, or call diversion, is a telephony feature of some telephone switching systems which redirects a telephone call to another destination, which may be, for example, a mobile telephone, voicemail box or another telephone number where the desired called party is available. Call forwarding was invented by Ernest J. Bonanno. In North America, the forwarded line usually rings once to remind the customer using call forwarding that the call is being redirected. More consistently, the forwarded line indicates its condition by stutter dial tone. Call forwarding typically can redirect incoming calls to any other domestic telephone number, but the owner of the forwarded line must pay any toll charges for forwarded calls. Call forwarding is often enabled by dialing *72 followed by the telephone number to which calls should be forwarded. Once someone answers, call forwarding is in effect. If no one answers or the line is busy, the dialing sequence must be repeated to effect call forwarding. Call forwarding is disabled by dialing *73. This feature requires a subscription from the telephone company. Also available in some areas is Remote Access to call forwarding, which permit the control over call forwarding from telephones other than the subscriber's telephone. VOIP and cable telephone systems also often allow call forwarding to be set up and directed via their web portals.

In Europe, most networks indicate that unconditional call forwarding is active with a special dial tone. When the phone is picked up it is immediately apparent that calls are being forwarded.
-- Wikipedia
Legal Definition
A telephone feature that allows calls made to one number to be forwarded to another specified number.