Microarchitecture
The concrete internal design of a processor — how the pipeline, the caches, the execution units, and everything else is actually built. It differs from the instruction set (ISA): the ISA is "what" the CPU can do, the microarchitecture is "how" it does it.
The distinction: the ISA (e.g. x86-64, ARMv9) is the contract — which instructions and registers exist, what programs see. The microarchitecture is a specific implementation of that ISA: how deep the pipeline is, how large the issue width is, how the cache is organized, how the branch predictor works. The same ISA can be implemented by many different microarchitectures with widely differing performance — all x86-64 CPUs run the same instructions, but AMD's "Zen 4" and Intel's "Golden Cove" are completely different microarchitectures internally. It's at the microarchitecture level that the competition and the general performance leaps happen (a new "uarch" each generation). Understanding the difference explains why "the same instruction set" doesn't mean "the same performance". A fundamental level of abstraction in computer architecture. Related to ISA and RISC vs CISC.