A series test is a type of test that involves multiple parts or stages. The term 'series test' can refer to any type of test that requires the completion of multiple tasks or stages in order to be successful. In this case, the crossword solution for the term 'series test' is 'PILOT, ' which refers to someone who operates an aircraft. A PILOT is responsible for the safe operation of an aircraft, and must complete a series of tests in order to become certified.