Parallel testing means testing multiple applications or subcomponents of one application concurrently to reduce the test time. Parallel tests consist of two or more parts (projects or project suites) that check different parts or functional characteristics of an application. These parts are executed on individual computers simultaneously. The ability to test more than one application part “in parallel” allows you to significantly reduce the test time and test your applications more efficiently.
Book: Disciplined Agile Delivery by Scott Ambler and Mark Lines.