Reorder buffer (ROB)
Beginner
A list that keeps every step in its original order, so results can be made final in that order even if they finished in a jumble.
Novice
A queue that holds every in-flight instruction in program order from dispatch until it retires. Instructions finish in any order but leave the reorder buffer only from its head, in order, which makes wrong guesses and errors easy to undo.
Expert
Circular buffer from dispatch to commit: tracks completion and exceptions, drives in-order commit, and gives precise exceptions. Its size caps how far ahead the core can speculate.
Explained in Branch prediction and out-of-order execution (Architectures).
See also: Out-of-order execution, Precise exception, Instruction window.