Flicker is the visual nuisance a lamp produces when its supply voltage fluctuates several times per second. The human eye is particularly sensitive around 8.8 Hz, where a voltage variation of barely 0.3 % is already noticeable.
It is caused by loads that vary abruptly: arc furnaces, welding, starting of large motors, mills. It does no damage to equipment, but it is one of the most frequent complaints from customers connected near industrial installations, which is why it is regulated.
It is quantified through two indices defined in IEC 61000-4-15: Pst, short-term severity, measured over 10-minute windows, and Plt, long-term, calculated from twelve consecutive Pst values, that is, over 2 hours. The conventional nuisance threshold is a Pst of 1.