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noun, often attributive \ˈtwī-ˌlīt\
Definition of "Twilight"
1: the light from the sky between full night and sunrise or between sunset and full night produced by diffusion of sunlight through the atmosphere and its dust; also : a time of twilight
So Twilight = time between Day and Night.
Twilight Sparkle's original name was Twilight Twinkle, as in "Twinkle twinkle, little star."
Princess Luna represents the moon, which comes out along with the stars at night. Princess Celestia represents the sun, which is itself a star. Twilight Sparkle is Celestia's trusted apprentice, and has been studying under her for years. She was also part of Celestia's plan to free Luna from being Nightmare Moon. This was accomplished by harnessing Twilight's friendships through her magic. So Twilight is bound up with Celestia through her magic, and with Luna through her friendship.
Twilight's magic manifested itself as a crown, while the other ponies only got necklaces.
Celestia and Luna are also the only beings in Equestria with both unicorn horns and pegasus wings. And while Twilight is a unicorn, in the flash game she is shown to have the ability to give others necklaces.
My conclusion? Twilight is being groomed to take over as the sole ruler of Equestria, taking both Celestia and Luna's places as goddess. Since she's representative of both day and night, her reign would heal the divide in the world that turned Luna into Nightmare Moon, and usher in an age of supreme harmony.
Twilight Sparkle is the pony messiah.