Tamper resistance
Tamper resistance is the set of physical and logical measures that make opening a crypto device expensive without the keys surviving.
A smart card or HSM is only as secure as its casing. Tamper resistance combines protective coatings, sensors for voltage, temperature and light, and routines that wipe the keys when someone tries to open the device. In FIPS 140, level 3 and above require that an intrusion is both visible and triggers key zeroization.
It buys time, not immunity. Fault and side-channel attacks can go around it by reading the chip from the outside, which is why the two are usually discussed together.