I think it is called “triplets”
You can find all details in 3GPP website, or for a more practical definition here is a wikipedia copy :
The AUC stores the following data for each IMSI:
* the Ki
* Algorithm id (the standard algorithms are called A3 or A8, but an operator may choose a proprietary one).
When the MSC asks the AUC for a new set of triplets for a particular IMSI, the AUC first generates a random number known as RAND. This RAND is then combined with the Ki to produce two numbers as follows:
* The Ki and RAND are fed into the A3 algorithm and a number known as Signed RESponse or SRES is calculated.
* The Ki and RAND are fed into the A8 algorithm and a session key called Kc is calculated.
The numbers (RAND, SRES, KC) form the triplet sent back to the MSC. When a particular IMSI requests access to the GSM core network, the MSC sends the RAND part of the triplet to the SIM. The SIM then feeds this number and the Ki (which is burned onto the SIM) into the A3 algorithm as appropriate and an SRES is calculated and sent back to the MSC. If this SRES matches with the SRES in the triplet (which it should if it is a valid SIM), then the mobile is allowed to attach and proceed with GSM services.
After successful authentication, the MSC sends the encryption key Kc to the Base Station Controller (BSC) so that all communications can be encrypted and decrypted. Of course, the mobile phone can generate the Kc itself by feeding the same RAND supplied during authentication and the Ki into the A8 algorithm.