Megaquartz movements were characterized by their use of dual crystals and a hybrid quartz/mechanical design. The two quartz crystals operated at different frequencies, 1 KHz and 16 KHz. This gave very high precision timekeeping. These movements were not thermocompensated.
Note that Jaeger-LeCoultre produced a similarly-named but unrelated hybrid quartz movement in the 1980's, the Jaeger-LeCoultre Mechaquartz.