A lower third is a partial overlay to a video stream that combines text and graphic elements to provide the viewer with contextual information. It is placed in the title-safe lower area of the screen. Note that a "lower third" does not necessarily occupy the full third of space on the screen, despite what the name suggests.
In its simplest form, a lower third can just be text overlaying the video. Frequently this text is white with a drop shadow to make the words easier to read. A lower third can also contain graphical elements such as boxes, images or shading. Some lower thirds have animated backgrounds and text.
Lower thirds can be created using basic home-video editing software or professional-level equipment. This equipment makes use of video's alpha channel to determine what parts of the graphic or text should be transparent, allowing the video in the background to show through.
Lower thirds are also often known as "CG" (from character generator) or captions, and sometimes chyrons in North America, due to the popularity of Chyron Corporation's Chiron I character generator, an early digital solution developed in the 1970s for rendering lower thirds. Other common terms include super bars (or simply supers) (US), name straps, and actions (after Aston Broadcast Systems) (UK).
Video with lower thirds is known as a program as broadcast or dirty. Video without lower thirds is known as a clean feed or textless. For international distribution programs often include textless elements on the master tape: these are all the shots that lower thirds and digital on-screen graphics have been applied to, placed end-to-end so engineers can make a clean master if necessary.