What are the levels of autonomy in autonomous vehicles?

The levels of autonomy are also called levels of driving automation and there are six of them from 0 to 5, which is from no automation to full automation without human intervention. These levels were introduced in the Society of Automotive Engineers (SAE) standard J3016 by SAE On-Road Automated Driving (ORAD) committee. The US National Highway Traffic Safety Administration (NHTSA) and German Federal Highway Research Institute (BASt) have also proposed the levels of autonomy that are slightly different from the ones from SAE.

Level 0 (No Automation): The human driver is responsible for all the driving tasks, which are steering, acceleration and braking. There are few features available for providing warnings and momentary assistance to the driver such as automatic emergency braking, blind-spot warning, and lane departure warning. However, the driver should take necessary actions considering these warnings.
Level 1 (Driver Assistance): The driver should be driving even if few support features such as lane centering (LC) or adaptive cruise control (ACC) are engaged and controlling the steering, acceleration, and braking for the brief period. The vehicles which have only one of these features (either longitudinal control (LC) or lateral control (ACC) features, but not both) fall under this level.
Level 2 (Partial Automation): In this level of automation, the features such as LC and ACC provide support to the driver in both longitudinal (acceleration/braking) and lateral (steering) control of the vehicle. Still, the driver must continuously monitor these support systems and take control of the vehicle when needed for maintaining safety.
Level 3 (Conditional Automation): At this level of automation, the driver is not driving when the autonomous features of the vehicle are engaged, but the driver should be seated in the driving seat and must take control when these features request so. The autonomous system has a limited set of scenarios in which it can drive safely and all the necessary conditions of these scenarios should be met.
Level 4 (High Automation): The vehicle having this level of autonomy does not require the driver to take over the driving task. This type of autonomous vehicle operates mostly in the known environment. Sometimes, in the case of local driverless taxis, the acceleration/braking pedals and steering wheel might not be even installed.
Level 5 (Full Automation): This is the ultimate level of automation where the driver is not driving and the autonomous system can drive the vehicle under all the driving conditions (such as lighting conditions (day/night), road conditions (highway/off-road), traffic(no/heavy), weather conditions (rain/snow), and so on).

Levels of driving automation (SAE International,2018)