Each complete video image in NTSC/PAL (frame) is composed of two fields. One field consists of the odd numbered lines in the frame and the other field consists of the even numbered lines. The viewed together in rapid succession forms the complete frame image. The NTSC and PAL formats called for double scanning fields to reduce the visual flicker that was common on early TV monitors.
In video, a Field is half of a video frame, used in interlaced scanning. Two fields create one complete image in older analog systems.