SIM cards are just smart cards, so apart from pointing to the generic definition of Tamper Resistant, I'm not sure much can be said in general.
Looking up the manufacturer specs may provide additional info. For example, Gemalto (a specialist in hardware crypto chips) manufacturers a large number of the world's SIM cards. Their marketing page says this:
With a microprocessor base, SIM cards are similar to a mini-computer with its own operating system, storage and built-in security features. When configured with up-to-date cryptography, as recommended by NIST, BSI, ANSSI, etc., SIM cards provide a level of security that is state-of-the-art.
That's all kinda vacuous and fluffy, bet it's reasonable to assume that SIM chips will contain similar security technologies to Gemalto's FIPS 140-2 certified products.
This excellent answer by @ThomasPornin is also relevant: