A backpack can be; it depends on whether it's your only item and whether it's small enough to fit under the seat.

A backpack or messenger bag are almost always treated as a personal item. The only time is if they're too large to be a personal item and get upgraded, or if you bring two items when they limit you to one personal item + one carry-on -- then they usually treat the smaller one as a personal item. It's generally safe to assume unless it's one of those 40L packs, it's a personal item, but check your airline's dimension and weight limits for personal items.