From AMS Glossary
An electromagnetic transient launched by a lightning discharge of large amplitude within the earth–ionosphere cavity.
Originally named by Toshio Ogawa (1967), the "Q" connotes "quiet" and characterizes a transient in which the fundamental 8-Hz mode of the Schumann resonances is the dominant contributor. In general, a mix of Schumann modes makes up a Q burst.
Ogawa, T., et al. 1967. Worldwide simultaneity of occurrence of a Q-type ELF burst in the Schumann resonance frequency range. J. Geomag. Geoelectr.. 19. 377, 384.