Parallel
Parallelism is when multiple computations are executed at a time. Therefore at any instant, multiple executions are ongoing simultaneously.
Parellel computing doesn't need multiple tasks. Even one task can be split up into multiple sub-tasks and be assigned to different cores of the processor.
The Art of Concurrency defines the difference as follows:
A system is said to be concurrent if it can support two or more actions in progress at the same time. A system is said to be parallel if it can support two or more actions executing simultaneously.