Interrupt vector table
A table at a known address where the CPU looks up which routine to run for each interrupt.
When a timer, a UART or a fault raises an interrupt, all the processor knows is a number. That number indexes into the table, and the entry holds the address of the interrupt service routine (ISR). The Intel 8086 put its table at address 0 with 256 four-byte entries; ARM Cortex-M puts it at the start of flash, where the first word is the initial stack pointer and the second is the reset vector — the very first thing executed after power-up.
Startup code and the linker script fill the table in, typically with a weak default handler that just spins forever. Forget to define a routine and the interrupt lands there, and debugging begins with the question of why the program has frozen. Relocatable tables (VTOR) let a bootloader and an application each have their own.