x86 is a generic term referring to the "instruction set" of the most successful CPU "architecture". The architecture is defined as how the CPU arranges and uses resources such as RAM and I/O. Intel invented the x86 architecture, and AMD, VIA, Cyrix and others have manufactured chips compatible with x86.

CPU''s read bytes from RAM as instructions and do stuff based on that data, this is how programming ultimately works. All computer languages eventually have to be translated into this data, called machine language. A byte is a number 0-255, and the set of byte values and their corresponding instructions forms the instruction set.

There are many different types of CPUs that adhere to a specific philosophy, and therefore each major type of CPU has its own instruction set that is not compatible with a different CPU. Other types of instruction sets include MIPS, PPC, ARM, 6502, Z80, 68000, and others.

The two main types of philosophies are CISC (Complex Instruction Set Computing) and RISC (Reduced Instruction Set Computing). CISC packs more "power" into each instruction but requires the CPU to be slower and more complicated, RISC makes each instruction do less but makes the CPU cheaper and faster. x86 is a CISC CPU (as were most in the late 70''s/early 80''s when x86 was born) with RISC extensions.

x86 as a name comes from Intel''s numerical designation various chips in the architecture. It begins with the 8086, and then 80286, 80386, 80486, and onward through the Pentium chips, Core line and beyond (often called "686" chips even though they no longer have numbers).

To summarize, x86 is a term referring to the way that the CPU in the common PC was built and how it functions.