USSD (Unstructured Supplementary Service Data) is a Global System for Mobile(GSM) communication technology that is used to send text between a mobile phone and an application program in the network. Applications may include prepaid roaming or mobile chatting.
USSD is similar to Short Messaging Service (SMS), but,unlike SMS, USSD transactions occur during the session only. With SMS, message scan be sent to a mobile phone and stored for several days if the phone is not activated or within range.
The Wireless Application Protocol (WAP) supports USSD. USSD is defined in the GSM standard documents GSM 02.90 and GSM 03.90.