Edward speaks on this topic specifically to Bella in middle of the book:
"It's twilight," Edward murmured, looking at the western horizon. . . . His voice was thoughtful, as if his mind were somewhere far away. . . . "It's the safest time of day for us," he said, answering the unspoken question in my eyes. "The easiest time. But also the saddest, in a way . . . the end of another day, the return of the night. Darkness is so predictable, don't you think?" He smiled wistfully.
So, there you go, the safest time. In some ways, the most beautiful and the saddest time. Edward's answer is cryptic, however. My opinion burgeons from here into the idea that vampires can't be seen in sunlight, . . . and perhaps night is when their greatest lust for blood surfaces. Therefore, Edward is correct in his assessment.
Also, at the prom, there is a point where Bella indicates that she thought Edward was going to change her that night (she had no idea he was taking her to the prom). He says to her that her life has barely started, and she shouldn't be ready to give that life up, or for this moment to become the twilight of her life.
I believe it is explained somewhere in the book as the time of day that is most easy for vampires, yet is also the saddest time of day, because it symbolizes an ending. Vampires, as immortal, are stuck somewhere in between life and death. They are forever stuck in "twilight," not in the sun or the daylight of life, yet not in the complete darkness of death. Rather they are stuck between, in the twilight of life, where everything fades to a dim grey.
This question should be on the discussion board because I think the significance of the title is more of an interpretation than a definitive answer.
Having said that, in my opinion, the title has to do with the fact that vampires like the Cullens are able to come out into daylight only on days when the sunlight is no brighter than it is during twilight--the minutes just before sunset and just before daybreak. That is why they choose to live in the northwest, where the skies are overcast most of the time. Unlike in most other vampire stories, the characters in "Twilight" do not burst into flame when exposed to sunlight. Instead, they have another unique feature that would expose them for being different from human beings.
I hope this helps!
For me the word Twilight has either signified the beautiful glow when the dun is about to fall and present a darker side of the world, the moon. I've also associated the word Twilight with a gradual fall from something like a job when used in regards to people. So in my eyes the title Twilight stands for Bella's gradual decline from the human world, and her self as well, into something more beautiful (the Cullen's) and dark (the world of the vampires).
in all of the above posts its usually logical but throwing light on the "literature" aspect v can c tht the title gives a soothing and romantic touch it signifies the inner story in a sense tht it explains the love of edward and bella, and also dat there love is not hot as daytime neither its cold as night but its sum whr in between calm, beautiful and soothing for each other.........