n. The rising light of the morning; the dawn of day, or morning twilight.n. [capitalized] In Roman mythology, the goddess of the dawn: called Eos by the Greeks.n. The aurora borealis or the aurora australis (the polar lights).n. A reddish color produced by dyeing with arnotto.n. In electricity, an electric glow-discharge, appearing at very high voltages and some what resembling the aurora borealis.n. The length of time elapsing between successive maxima or minima of auroral displays, as the diurnal, the annual, the sun-spot, and the 55-year period. The last is possibly equivalent to 5 sun-spot periods. See auroral periodicity.