Optimizations which can be applied to a program without grokking the whole thing. These optimizations usually involve rearranging or rewriting sequences of common instructions in simple ways to make them faster or smaller. A simple x86 example: the obvious way to store zero in a register is MOV 0, EAX but it's actually faster to use XOR EAX EAX. Peephole optimizers are quite simple to write and maintain, and can easily win 20-40% more speed.