Multiple threads taking turns executing on the same hardware resource. Concurrency creates the illusion of simultaneous execution through interleaving, but only one thread runs at any given instant on a single core.
Context
Concurrency is essential in operating systems and server design. For example, a web server handles thousands of requests concurrently by switching between them, even on a single CPU core. This differs from parallelism, where multiple threads truly execute at the same time on different cores.