Edge- and level-triggered interrupts
The two ways an interrupt line can signal: with a change (edge) or with a state that persists (level).
An edge-triggered interrupt fires when the signal goes from low to high, or the other way, and is then forgotten. That suits buttons and pulse counters, but it has a classic trap: two edges arriving while the interrupt is masked register as one, and an edge that arrives before the handler acknowledges can vanish altogether. A level-triggered interrupt stays asserted for as long as the device holds the line. Nothing gets lost, and several devices can share one line — which is why PCI chose levels.
The price of levels is that the handler must silence the source before returning, or the same interrupt fires again immediately and the CPU does nothing else. Linux epoll borrowed both terms for event loops, traps included.