The TIMSI (Temporary IMSI) is a pseudo-random number generated from the IMSI (International Mobile Subscriber Identity) number.
The TIMSI is utilized in order to remove the need to transmit the IMSI over-the-air. This helps to keep the IMSI more secure.
To track a GSM user via the IMSI/TIMSI, an eavesdropper must intercept the GSM network communication where the TIMSI is initially negotiated.
In addition, because the TIMSI is periodically renegotiated, the eavesdropper must intercept each additional TIMSI re-negotiation session.