Nighttime Equatorial Ionization Anomaly (EIA) and plasma irregularities have been a major focus of the equatorial- and low-latitude ionospheric research community. The behavior of the daytime ionosphere-thermosphere system, geometry (such as magnetic declination angle, terminator alignment) and other conditions (e.g. winds and wave activity) play vital roles in the morphology and variability of the nighttime EIA and ionospheric irregularities. Determining these conditions and understanding their interactions have challenged the research community for decades. Furthermore, new satellite measurements (GOLD, ICON, COSMIC) have revealed several new, interesting characteristics about the EIA variability and ionospheric irregularities. This session invites presentations related to space and ground-based measurements and modeling efforts that contribute to a better understanding of the development and variability of nighttime EIA and ionospheric irregularities over equatorial- and low-latitudes.