Concurrent usually refers to any kind of control flow in which separate (logical) threads of execution can be interleaved, and where the particular interleaving

Concurrent usually refers to any kind of control flow in which separate (logical) threads of execution can be interleaved, and where the particular interleaving can affect the final result. For example, event systems are inherently concurrent, even if they are single threaded: the order of events affects normally affects the final state of the system (for example, if you receive a request to SET X=0 and a request to INCREMENT X, the final result may be 0 or 1 depending on the order, even if you have a single thread of execution actually handling the events). In contrast, parallel programming usually refers to program flows where multiple instruction streams can be interleaved arbitrarily without changing the final result. For example, when using mapping a pure function on a list, the execution of the function for each element of the list can be executed in parallel without any concurrency. Alternatively, parallel execution can refer to any case where multiple instruction streams are actually executed in parallel, regardless of their order. Concurrency often has pitfalls even with single-threaded execution models. For example, if two generators share some common state, even if they are executed in a single thread, the final result depends on how they are used and may be unexpected.