Instruction set architecture (ISA)
Beginner
The list of commands a kind of chip understands, and exactly what each one does. Programs are written for it, and chips are built to follow it.
Novice
The contract between software and hardware: the instructions, registers, data types and memory behavior a program may rely on. Any chip that implements the same ISA runs the same programs. x86, Arm and RISC-V are ISAs.
Expert
The architectural state and the semantics of every instruction, including exceptions and memory ordering. Timing is deliberately left out, so microarchitectures can differ. The IBM System/360 (1964) first separated the two.
Explained in Instructions and pipelines (Architectures).
See also: Architecture and microarchitecture, RISC and CISC, RISC-V.