It is debatable because not all publications have the same definitions. One of the easier definitions is as follows.
1st order bends occur in the same plane as the archwire when placed flat on a table. Ultimately, after the bends are complete, the archwire remains flat laying on the table. They could be step outs, step ins, or z-bends producing rotations in this plane.
2nd order bends result in the archwire no longer being able to lie completely flat on a table. They are step ups, step downs, and z-bends producing tip.
3rd order bends are twists in the archwire producing torque.