The absolute truth is that there is no one meaning for the term twin flame.
Some say it is a soulmate. Others say it is a kindred spirit. Still others say that it is a being who was incarnate at the same time that you were, from the same life spark that created your everlasting soul. Some say you are incomplete without your twin flame in your life. Others say that your twin flame is never incarnate when you are. Still others say you have more than one twin flame, while yet others say that you only have one twin flame and it is the purpose of your life to seek out this being and unite your souls.
Some astrologers say that you can determine who your twin flame is by examining the birth charts. Other people say that twin flames are never to be your life partner, but rather, they are in your life to teach you some of your greatest lessons, and to help you find why you took this incarnation.