Out-of-order execution
Beginner
When a processor runs the steps of a program in whatever order their inputs are ready, then puts the results back in the right order.
Novice
A way of running a program in which the processor executes each instruction as soon as its inputs are ready, rather than strictly in program order, and then commits the results in program order so the program sees no difference.
Expert
Dynamic scheduling of a window of renamed instructions in dataflow order, with in-order commit through a reorder buffer. It hides latency (cache misses, long operations) by finding independent work, bounded by window size, issue width and branch prediction.
Explained in Branch prediction and out-of-order execution (Architectures).
See also: Reorder buffer (ROB), Register renaming, Instruction window.