Interrupt
Signal to the CPU to pause current work and handle something urgent — keypress, packet from NIC, timer tick.
Two categories: hardware interrupts (IRQs from devices) and software interrupts (syscalls, exceptions). CPU saves registers, jumps to handler, handles it, restores. Polling is the alternative — cheaper when events are extremely frequent (DPDK avoids interrupts on datacenter NICs). Spurious interrupts, interrupt storms are debugging nightmares. APIC + MSI on modern x86 gives 256+ IRQ lines.