A high-level but not super useful definition:
A precise, architecturally visible definition of shared memory correctness.
A more useful but still intangible definition:
A specification of the allowed behavior of multithreaded programs executing with shared memory.
More concretely:
The rules of how reads and writes to different memory locations are observed by other processors.
I think it really helps to see the definition of memory consistency multiple times—this topic makes head hurty.
A processor provides memory consistency by adhering to a memory consistency model.