AKA "offline", "radios off", or "standalone" mode
Some phones and other wireless devices have a special "flight" or "airplane" mode that turns off just the wireless radio parts of the device, for safe use on an airplane where radio transmitters are not allowed.
Most airlines and many governments ban the use of wireless radio devices during flight. Although preventing interference with the plane's onboard systems is the most well-known reason for such bans, the more important concern with cell phones is interference with cell phone networks on the ground, which cannot handle phones at high relative altitudes and moving at airplane speeds.
Airplane mode therefore allows the user to safely use the non-wireless functions of a phone (such as music, games or organizer functions) on an airplane during flight.
However, not all airlines have policies that accommodate devices with airplane mode; some airlines still ban the use of all phones during flight regardless.
There is a special industry-standard flight mode icon, although many phones use a non-standard icon.
See: Flight Mode Icon