Twilight Squadron was a Rebel Alliance military unit. It was one of the most reputable units of the infamous Eclipse Team.
Three of the five members of Twilight Class who founded Eclipse Team—Adazian Liebke, Ma'w'shiye and Sisquoc (who was the commander of Twilight Squadron)—later joined Twilight Squadron.
The Galactic Empire had its own Twilight Squadron, but all of its members eventually defected to the Rebel Alliance. They were reassigned to other units.