means The soft, dim light in the sky between full daylight and full darkness, especially after sunset.

from An Old English compound: twi- meaning "two" or "half" (a cousin of "twin" and "between") joined to "light." The "twi-" piece is the puzzle — some read it as "half-light," others as the "two" lights of dusk and dawn — but the sense has always hovered in that in-between hour. The same "twi-" haunts "twilit" and lurks, faded, in words like "twice."