Abigail is Taylor's childhood friend from the highschool she went to in Nashville, Tennessee, when she moved there to pursue her dreams of writing country music. Abigail's name has been featured in the song Fifteen from Fearless, as well has been publicly Taylor's close friend all the way up to this day.
Although the song is not specifically actually about Abigail, her name is explicitly mentioned as being part of the story Taylor is telling... "Abigail gave everything she had to a boy who changed his mind, and we both cried"