Instruction Set Architecture
An instruction-set architecture (ISA), also known as an instruction set or just an architecture. is a specification of what instructions a processor understands. Lots of processor models will use the same ISA, though later models often add extensions by which programmers can use their extra capabilities. For example, x86 is an ISA designed by Intel for their 8086 processor and used by its successors to this day. Motorola 68000 (m68k) is another. MMX and SSE are examples of extensions added to more recent x86 processors. From the programmer's point-of-view, an "instruction-set architecture" and an "assembly language" are pretty much the same thing, though technically the ISA refers to the machine code rather than the text-based assembly language used to produce it.
There are also ISAs that do not run on actual hardware: Java virtual machine code is an example of this.
For more information, see Wikipedia.