Brown-out
A supply that sags below what a circuit needs without going to zero — the half-alive state where logic misbehaves, resets loop, and memory corrupts. Named for the way incandescent lights dim brown during one.
In practice
Designs that only work at nominal voltage brown out at the bottom of the range: cold batteries, long cables, shared supplies under load. Microcontrollers have brown-out detectors for a reason — enable them, and check the worst-case rail math before the field does.