1. <hardware> A video display system which builds an image on the VDU in two phases, known as "fields", consisting of even and odd horizontal lines.
The complete image (a "frame") is created by scanning an electron beam horizontally across the screen, starting at the top and moving down after each horizontal scan until the bottom of the screen is reached, at which point the scan starts again at the top. On an interlaced display, even numbered scan lines are displayed in the first field and then odd numbered lines in the second field.
For a given screen resolution, refresh rate (frames per second) and phosphor persistence, interlacing reduces flicker because the top and bottom of the screen are redrawn twice as often as if the scan simply proceded from top to bottom in a single vertical sweep.
2. <graphics> progressive coding.