Multi-threading
What does multi-threading mean? It does not mean that the core executes more than one thread at a time! Instead, we use this term when the operating system has a list of threads that could eventually run; some of these can be suspended for a while because they need data that is not available yet (user input, data from another thread); others will be ready for execution but are not considered important enough yet. And then there is one thread that is actually executing.
When the OS switches transparently from one thread to another and does it fast enough, we perceive this as if more than one thread is running at the same time.