With regards to GSM phones, tethering refers to the process of connecting a mobile phone to a PC so the PC can access the web by means of an EDGE/UMTS/HSDPA network. Through this means the phone acts as a modem for the PC.
That particular utilization of tethering originates from the way that it is to be used as a modem, the phone must be connected with the PC by means of a USB cable link. However due to the advance of technology this same process can be done using the Bluetooth connection or the Hotspot.
Therefore, tethering has transformed into a conventional term for sing your phone as a GSM modem for your PC.