Tethering

Definition
Tethering allows sharing an internet connection of a phone or tablet with other devices such as laptops. Connection amongst mobile devices can be done over WiFi, Bluetooth or by using a cable, for example USB. If tethering is done over WiFi, the feature is called Mobile Hotspot. The mobile device connected to the internet can thus act as a portable wireless access point and router for other mobile devices connected to it.

Availability
Tethering is possible on:
 * Android phones from version 2.2
 * BlackBerry devices from OS 7.1.
 * iPhone 4, iPads 3rd gen. or later with iOS from 4.2.5 or later
 * Windows Phone from OS 7.5 or later
 * for other devices there are third-party applications to allow this

Tethering and mobile providers
Some providers don't like tethering very much or like to charge extra for using it. Some others ban it in their T&Cs. But most of the providers nowadays allow tethering, because it needs some effort to really distinguish tethered data from non-tethered. Only very few operators sniff and block it. Where a special policy is known, it's stated in the respective chapters. But even when it's blocked like on many Lycamobile plans, you can still use it in different mobile devices, but not simultanously.

Tethering and iOS devices
While Google (Android) and Microsoft (Windows Phone) always enable tethering on their devices, Apple (iOS) is more restrictive with its phones and tablets. Here a special profile for tethering needs to be activated and installed. This can only be done, if the mobile network operator releases an official profile for tethering on the iPhone and iPad. Thus mobile operators can much more effectively control on Apple devices whether you can tether on their network or not.

So restrictions for Apple products on tethering can't be generalized to other systems. Generally, 3rd party apps can circumvent any lock-out, but for this your device needs to be rooted and/or jailbroken.