Always on Display

Always On Display (AOD) is a smartphone feature that shows limited information while the phone is asleep. It is widely available on Android handsets. On some Android phones, the feature is called Ambient Display or Active Display depending on its implementation and behavior. Depending on the phone's design, it may be a replacement feature or complementary to the Notification LED.
Overview.
A phone with AOD enabled keeps a limited portion of the screen on during sleep mode. Instead of the notification LED blinking to serve as an indicator for any incoming messages or notifications which the user may need to check out, an Always On Display shows the time, date, and battery status by default, but can be configured to also show various types of notifications as they arrive or screensavers.
All current Android phones have differing behavior for this feature. Some phones briefly turn on the Ambient display when notifications arrive and then the screen turns off. In some models, the screen is off to save battery, turns on when notifications arrive and remains on as long as they are not dismissed or read. Depending on whether the manufacturer supports it, not all apps may be supported for showing notifications with this feature - only first party apps or popular apps may be supported, or all third party apps may be supported but may need to be explicitly enabled.
Always On Display/Ambient Display feature can also be configured in some phones to wake up with a double or single tap, a hand gesture or picking up the phone.
History.
This technology was first introduced by Nokia in 2009 on the N86, and more widely adopted with its next generation AMOLED Symbian phones in 2010 (the Nokia N8, C7, C6-01 and E7). It became a standard feature on most Nokia Lumia Windows Phones in 2013, paired with the Nokia Glance Screen app. The feature has since become more widely available on Android handsets including Huawei (Mate 10 Pro, P20 Pro), Motorola (Moto X, Z, G), LG (G5, G6, G7, V30, V35, V40, V60), Samsung (Galaxy A3(2017), A7(2017, 2018), A30, A50, S7, S8, S9, S10, S20, S20 FE, S21, Note FE/8/9/10/20) and Google Pixel (Pixel 2, Pixel 3, Pixel 3a, Pixel 4, Pixel 4a).
Battery impact.
The Always On Display feature does consume energy, although the Samsung Galaxy S7 and S7 Edge phones, and later phones that made the feature popular again are built with AMOLED screens that turn off black pixels. On today's AMOLED phone displays, it is true that only a few pixels may need to be turned on but they do need to be moved to prevent pixel burn in. Colors, sensors and processors all consume energy while AOD is in use, which leads to an extra consumption of roughly 3% battery.
Also on LCD displays, the backlight has to be turned on even if a part of the screen shows information so this feature consumes a significant amount of power compared to a notification LED.
Typically, an Ambient display solution which turns on the screen only when notifications are present, remains on, but turns off when they are dismissed will consume the least amount of battery power while still drawing the user's attention when required, in contrast to an Always-on Display which will keep the screen on, all of the time, to show some information, even if notifications may not be present. Since the date and time are less essential than battery status or notifications which may require the user's immediate attention, an AOD can be customized in many app-based implementations to only show notifications or selectively choose what is shown.
Scheduled on/off times.
In some phones, Always On Display/Ambient Display feature can be set to be turned on, on a schedule and turned off, for example, at night or when in the pocket, with help from the proximity sensor. Also, some phones keep the screen off most of the time but light up only for notifications and nothing else, and once the notifications are acknowledged or dismissed by the user, the screen goes off again.