Backend & systems·Concurrency and coordination
your service is handling four hundred requests at once on eight cores, interleaving them whenever one is waiting on I/O.
Concurrency
Dealing with more than one task in overlapping time periods — a structure, not a hardware property. A single core can be highly concurrent, because most tasks spend their time waiting. Concurrency is what creates the need for locks and ordering guarantees; whether the tasks literally run simultaneously is a separate question.