Answer:
Step-by-step explanation:
Multi-core is having multiple processing cores on the same chip, essentially multiple CPUs on one bit of silicon. To be considered multi-core each core should be essentially a full CPU - the fact that even the earliest Pentium chips had multiple integer calculating units (allowing more more efficient pipe lining) does not count.
You could of course have multiple-processor multi-core arrangement, with more than one multi-core processor in the same machine.
Cooling: a two core CPU produces much less waste heat than two individual units of the same species and needs only one core heat sink and fan, typically cheaper (though all the heat is in one place, not spread over two, which may require higher-tech cooling solutions)
Cache speed: On the same chip you have the possibility to make the coherence/sharing of cache L2 (or L3) more successful as cores do not have to communicate on the external Memory Bus for a longer distance.
Simplicity cost differences: The multi-core approach does not require many motherboard sockets and so on.