Vertical blanking interval. The scan lines in a television signal that do not contain picture information. These lines are present to allow the electron scanning beam to return to the top and are used to contain auxiliary information such as closed captions.
Vertical Blanking Interval. A portion of the television signal that does not contain visual data. In NTSC, the VBI are lines 1 through 21 in each field.
This is the period of time between 'frames' in a video broadcast when information is still being broadcast but when the information is not being displayed on the screen. It contains many types of information such as teletext, color signal analysis etc.