Concurrency is about independent computations that can be executed in an arbitrary order with the same outcome. The opposite of concurrent is sequential, meaning that sequential computations depend on being executed step-by-step to produce correct results.

Concurrency can be limited or constrained by depending on communication or interaction between computations, and minimizing dependencies while preserving correctness is the central problem of concurrency modeling. The requirements for communication are derived either from modeling real-world dependencies between concurrent processes, or from the need for efficiency and reusing limited resources.

Concurrent programming means factoring a program into independent modules or units of concurrency. Depending on the context, there are different terms for units of concurrency, like tasks, coroutines, processes, threads or actors.

The wide relevance of concurrency is based on the need to map independent processes in the real world to computational models, and on the hardware trends towards increased parallelism and the rise of distributed computing and networking.