Contrasting backpacking, day and urban hiking comes down to where and for how long. Overnights or not. Wilderness or town.
Backpacking
Multi-day hikes with at least 1 overnight in the wilderness.
Day hikes
Leave and come home the same day in the wilderness. Carbon karma guideline: for every 1 hour in car, 3 hours hiking.
Urban hikes
Walking out the front door for an urban hike and returning 4-6 hours later. Hike typically over bike / walking paths and sidewalks.