ISA

The full set of instructions a particular kind of CPU understands — its native vocabulary.

An Instruction Set Architecture (ISA) defines exactly which operations a processor family supports and how they're encoded — x86, ARM, and RISC-V are different ISAs. Any code that runs natively must ultimately be expressed in the target's ISA, which is why a compiler or JIT needs a separate backend per architecture. It's the contract between software and silicon.