Answer:
I think of a hairline fracture as a non-displaced fracture. In other words, the bones are not moved out of position, but the involved bone is still broken, technically.
Usually, hairline fractures are tender to the touch and cause pain at the extremes of movement.
They may not be painful with gentle motion and there may be just a little swelling in the injured area.
Sometimes plain x-rays will not show a break and advanced imaging procedures like CT scan or MRI scan may be necessary to diagnose the problem.