Polling
Repeatedly reading a status register to see whether something has happened, instead of letting the hardware announce it with an interrupt.
The textbook pattern is while (!(UART->SR & RXNE)) {} followed by reading the byte. It is simple, deterministic and needs no ISR, which is why it shows up in bootloaders and in any superloop that visits every peripheral on each pass anyway.
The cost is CPU time and power: a processor that polls can't drop into a low-power mode, and events that land between two checks wait their turn. Ironically, polling wins at very high event rates — once interrupts arrive faster than they can be serviced, the per-interrupt overhead becomes the bottleneck, which is why network drivers switch to polling under load.