How do you define a core by instructions ?
Ok, let's take a step back.
Instruction is a specific term, not a generic one. There is a finite set of instructions you can feed to CPU that it is able to process called
instruction set.
An example - MOV is a specific instruction in
x86 instruction set that moves data from one location to another.
Extremely simple example - a calculator has instruction set of 4: addition, division, multiplication and division (lets assume no memory function or anything).
CPU is the piece that carries out these instructions.
From first example - x86 CPU needs to be able to do MOV. I will use the example above again - MOV with two registers as operand will use an ALU (part of execution stage and unit) and takes one cycle. MOV with register and a memory location as operand will use an ALU and AGU and will take a couple of cycles to complete.
Calculator example is simpler - these 4 instructions can be fed into execution stage pretty much directly, minor circuitry for fetch and no need for decode. Output can be fed directly to screen buffer.
When it comes to multicore CPUs, these are still defined based on a CPU and this is done via multiprocessor systems. Multiprocessor systems are simply what the name says - systems with multiple processors that are connected together. For multicore CPUs, the important part of these is mainly the homogeneous (in this context, same ISA) systems where multiple CPUs are in the same system connected together with a single bus. As time went by and technology evolved, the physical implementation has changed to put this onto a single die but both definition and principle are still the same. The only noteworthy addition to terminology is that a CPU in such situation is now called a core.
A note on that calculator example - the implementation of that instruction set is basically an ALU. Computer design course starts with building one pretty early on from gates and sometimes transistors, usually without the division though as that is a bit more complex to do. Although you physically need addition and multiplication - division is slight extra bit to addition circutry.