eFuse
A microscopic fuse in the silicon that can be blown exactly once and never restored — the chip's permanent memory for things that must not change.
IBM introduced the technique in 2004. A tiny conductor is driven with enough current that electromigration alters its resistance, and the value read back flips from 0 to 1 for good. A chip may carry hundreds or thousands of these bits, grouped into blocks written at the factory or later in the field.
They are used precisely where irreversibility is the point: unique chip IDs, calibration values from production test, flash encryption keys, a bit that disables JTAG permanently, and anti-rollback counters. On the ESP32, eFuses govern both secure boot and flash encryption. Anyone who blows the wrong bit by accident learns that "one-time programmable" is meant literally.