A microarchitecture is a hardware implementation of an ISA (instruction set architecture). An ISA is a structure of commands and operations used by software to communicate with hardware. A microarchitecture is the hardware circuitry that implements one particular ISA.
For example, x86-64 is the ISA used by most modern laptop and desktop computers. It is implemented by various microarchitectures, including those designed by Intel and AMD. Software that is compiled for the x86-64 ISA can run on any microarchitecture designed to use the x86-64 instruction set.
Multiple CPU models may be designed for a particular microarchitecture. For this reason, a microarchitecture is sometimes referred to as a "family" or "generation" of CPU. For example, Intel Kaby Lake (7th generation) and Coffee Lake (8th generation) are separate microarchitectures, each with their own "family" of compatible CPUs.
The word "microarchitecture" is sometimes abbreviated µarch. The Greek letter µ ("mu") is the scientific abbreviation for "micro." Because this letter does not appear on some keyboards, the abbreviation uarch may also be used.