IRQ
The signal from a hardware device saying "I need attention now" — an interrupt request.
Rather than have the processor ask every device whether it has anything to report, devices are allowed to interrupt. The network card raises one when a packet arrives, the disk when a read completes. The processor suspends what it was doing, runs an interrupt handler, and resumes.
Older PCs had fifteen physical IRQ lines, and two cards colliding on the same number was a familiar installation misery. Modern systems use MSI-X, where the device writes a message over PCIe instead of pulling a physical wire, giving thousands of distinct interrupts and letting a network card spread its queues across CPU cores. The distribution is visible in /proc/interrupts.