Interlaced scan is one of two methods used for “painting” an image on a television screen — the other being progressive scan.
How Interlaced Scan Works
Designed for the analog NTSC television system, interlaced scanning uses two fields to create a frame. One field contains all the odd lines in the image, the other contains all the even lines of the image. A television scans 60 fields every second (30 odd and 30 even). These two sets of 30 fields are combined to create a full frame every 1/30th of a second, resulting in a display of 30 frames per second. Drawbacks to interlaced scanning compared to progressive scanning include flicker, lower resolution and quality issues.
Compare With Progressive Scan
Progressive scan (also known as known as 480p) is where the lines are drawn in one at a time in sequential order. The entire single frame image is painted every 1/60th of a second, allowing for twice the detail to be sent in the same amount of time used in interlaced systems.
Recommended Reading: Webopedia’s progressive scan definition.