In video, a sample of the colour sub-carrier inserted into the horizontal blanking interval at the start of each line of video. A very accurately phased burst of high frequency at the beginning of each scanning line. This determines the colour of the signal.
Eight to nine cycles of colour subcarrier (at 3.58 MHz for NTSC) keyed onto the back porch of every horizontal sync pulse. This signal is used for colour phase reference or synchronization for the colour information (U and V or Y and Q signals) in the video line.
The colour burst signal amplitude is 20 units below and 20 units above the blanking level for a total of 40 units although, in modern circuitry, burst level is not critical. Burst phase is detected by calculating using the phase angle of the burst as it crosses over blanking.
Kodak: A Concise Glossary for the Non-Technician