Widget

A widget is an that is a feature of iOS, iPadOS, and macOS.

macOS
Widgets were first introduced as part of Dashboard in Mac OS X 10.4 "Tiger" and were discontinued with macOS 10.15 "Catalina". Widgets are being re-introduced in macOS 11 "Big Sur" in a Control Center that resembles iOS and iPadOS.

iOS
Starting with iOS 14, widgets can be placed anywhere on the home screen that can accommodate the widget's size. Widgets in iOS 14 can be displayed in 4 sizes: Small, which looks like a small tile, Medium, which looks like a square, and Large, which looks like a rectangle. There is also an Extra Large size for some apps like Weather and News.

Smart Stack is a feature in iOS 14 that allows quick scrolling through pre-selected widgets within the confined Today View space. Swiping up and down will let you change the displayed widget.

iPadOS
As of iPadOS 14, widgets cannot be placed in the home screen, but can be accessed in Today View. This can be accessed by swiping left, like in iPadOS 13.

Customization
The free 3rd-party utility My Widgets allows the creation of custom widgets for iOS and iPadOS.