What is a vehicle 'blind spot', and how must a driver check it before changing lanes or turning?

1. An area around the vehicle not visible in the rearview or side mirrors; check by glancing over your shoulder
2. A section of road where streetlights are non-functional; check by flashing high beams
3. The area directly under the front bumper; check by leaning forward over the steering wheel
4. A defect in the vehicle's windshield glass; check by cleaning with wiper blades