Retirement
The final step in an instruction's life in the CPU, where its result is made permanent and visible in the architectural state — in strict program order. An instruction "retiring" means it has officially completed and can't be undone.
Mechanic: in an out-of-order CPU, instructions have finished executing out of order, but their results are provisional until they retire. Retirement happens from the top of the reorder buffer (ROB) in program order: when the oldest instruction is done and error-free its result is "committed" to the architectural state (the real registers, memory) and it leaves the ROB. Why the order matters: it guarantees precise exceptions (if instruction N causes a fault, you know exactly that everything before N is done and nothing after N has affected the state) and correct rollback on misprediction (everything after the fault point is discarded before it retires). The retirement width (how many instructions can be committed per cycle) is a measure of the CPU's capacity. The end point of the out-of-order machinery. Related to reorder buffer and out-of-order execution.