Definition
x86 is a family of instruction set architectures (ISAs) descending from Intel’s 8086 processor, characterized by variable-length instruction encoding and decades of dominance in personal computers and servers. Today “x86” usually means its 64-bit extension, x86-64 (AMD64).
How it works
x86 defines the contract between software and hardware: how instructions are encoded and how registers and memory behave. One x86 binary runs on any chip that implements the architecture, which is what sustains its compatibility ecosystem; Intel, AMD, and other vendors extend and cross-license the architecture over time.
Usage
Windows and Linux desktops and laptops, most cloud VM instances, and a large installed base of enterprise servers remain x86. Vast amounts of legacy software ship only x86 binaries, which keeps compatibility layers alive.
Related and confusable terms
x86 is one instruction-set architecture for a CPU; the other major family is Arm, which dominates phones and has been expanding into laptops and servers. x86 is the software contract; the CPU is the hardware that executes it.