I think I would prefer the word aliased instead of overlayed. For me:
aliased = two different instructions that may access the same memory cell
overlayed = two different sets of memory that can be selected, i.e. basically banking.

And in C, an aliased variable access routes to the same variable (multiple pointers or mixing of direct and indirect adressing)

Most programming languages who has overlay handling, considers that swapping of different memory blocks into a common address range, i.e. a precursor to the swapping in a virtual-memory system.