I'm talking about ones where the margin is less than 7 points, or has swung by more than 5 points in polls within the 2012 results and polls currently in that state, or is trending way too much to one party and can flip from GOP to Dem or vice versa.
It differs, there's no true swing states, but depends on predictions. In different elections, the states can vary.
Swing state is defined as:
a US state where the two major political parties have similar levels of support among voters, viewed as important in determining the overall result of a presidential election.
So, in this unconventional election, the states that have exceptionally close polling numbers are Arizona, Iowa and Ohio where both candidates have changed leads throughout the election.
Though Hillary Clinton has hinted that she would compete in Texas, I do doubt that it's a swing state, it's still considered Republican-leaning and Trump is still a favourite to win.
Traditionally, swing states will include Colorado, North Carolina, Pennsylvania, Florida, New Hampshire and Nevada. However, the polling in these states doesn't seem to be close so far and there's a clear lead in each of these state.
You can check out a feature by The New York Times regarding this.