synchroscope

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An instrument that indicates the degree to which two alternating current systems are synchronised with each other; used especially in electricity generation
• noun An instrument used to synchronise the propellers of a twin-engine aircraft

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an instrument that indicates whether two periodic motions are synchronous (especially an instrument that enables a pilot to synchronize the propellers of a plane that has two or more engines)