A program is said to be concurrent if it may have more than one active execution context — more than one "thread of control". Concurrency has at least three important motivations:
In general, we use the word concurrent to characterize any system in which two or more tasks may be underway (at an unpredictable point in their execution) at the same time. Under this definition, coroutines are not concurrent, because at any given time, all but one of them is stopped at a well-known place. A concurrent system is parallel if more than one task can be physically active at once; this requires more than one processor. The distinction is purely an implementation and performance issue: from a semantic point of view, there is no difference between true parallelism and the "quasiparallelism" of a system that switches between tasks at unpredictable times. A parallel system is distributed if its processors are associated with people or devices that are physically separated from one another in the real world. Under these definitions, "concurrent" applies to all three motivations above. "Parallel" applies to the second and third; "distributed" applies to only the third.
Reference: [SCOTT] pp. 575 & 576.