What is the significance of the title: Twilight?

What is the significance of the title: Twilight?

Ok with a little help from Google to get me started, i read a little bit for an idea or two but i will endeavor to answer these questions on my own accord.

He ignored me, staring up at the moon.
”Twilight, again,” he murmured. “Another ending. No matter how perfect the day is, it always has to end.”
”Some things don’t have to end,” I muttered through my teeth, instantly tense. (pages 494-495)
……………………
”So ready for this to be the end,” he murmured, almost to himself, “for this to be the twilight of your life, though your life has barely started. You’re ready to give up everything.”
”Its not the end, it’s the beginning,” I disagreed under my breath. (page 497)

So what is twilight, according to Ninja Words.

twilight

noun

• The soft light in the sky seen before the rising and (especially) after the setting of the sun, occasioned by the illumination of the earth’s atmosphere by the direct rays of the sun and their reflection on the earth.

"I could just make out her face in the twilight."

• °The time when this light is visible; the period between daylight and darkness.

"It was twilight by the time I got back home."

adjective

• Pertaining to or resembling twilight.

"O’er the twilight groves and dusky caves. —Alexander Pope."

So its the time between day and night or between night and day. It is the time where day/night ends and night/day begins. In the last passage i read from the book, the whole twilight idea is summed up right there. Edward asks her if she really wants to end one life and to start another. She is in the twilight of her life. The whole book leads to this one decision that will change her life.