'Glow' is Hatch's term for the electric children due to the fact that they all glow (except for Nichelle). There are seventeen electric children. In The Prisoner of Cell 25, the glow is said to be very faint and only noticeable in darkness.
Vote for your favorite electric here: Top 10 Electrics
Currently there are only 16 Living glows out of 17 ( 1 is dead, 2 are loyal to Hatch and the Elgen, 3 are Rebel Glows and the other 10 are Electroclan)