I don't really see a difference between hiking and backpacking. Both are walking carrying your essentials and maybe a few luxuries.

Some say backpacking is longer duration, so does that mean that an overnight is a backpacking trip and no longer a hike? Then what is day hiking?

If I were pressed to give a difference I'd say that hiking might be more mobile while back backing would include hiking to your campsite and staying there for a few days.

To me it's essentially walking on trails in wooded/desert/rocky/not urban areas, usually with some necessities. A walk through the woods is on the edge between the two.