IMSI (International Mobile Subscriber Identity) — Definition

The IMSI, or International Mobile Subscriber Identity, is a unique number stored on your SIM that identifies you to the mobile network.

It begins with the MCC and MNC that identify your country and carrier, followed by a number unique to your subscription. The network uses it behind the scenes to recognise and authenticate you, and it's kept private to help prevent tracking.

In practice

You won't normally enter or even see your IMSI; it works silently. It matters mostly as the value that ties your SIM to your account on the carrier's systems. The MCC and MNC at its start are the parts you'll actually encounter, in your APN settings.