Reset vector
The address the processor fetches its very first instruction from — or a pointer to it — when it comes out of reset.
Every architecture fixes it in silicon. The 6502 reads a pointer from $FFFC. The 8086 started executing at FFFF0h, sixteen bytes below the top of its address space, and modern x86 still begins at 0xFFFFFFF0 — where the firmware flash holding the BIOS or UEFI is mapped, and where there is realistically room for one jump.
Cortex-M is tidier: the word at offset 0 of the vector table is the initial stack pointer and the word at offset 4 is the reset vector, so the core can be running C almost immediately after the startup code. The VTOR register lets a bootloader relocate the table before jumping into the application.