Interlaced scan is a display signal type in which one-half of the horizontal pixel rows are refreshed in one cycle and the other half in the next, meaning that two complete scans are required to display the screen image.
The i in a TV signal specification such as 1080i stands for interlaced scanning. The number indicates the number of horizontal lines in a raster. In an interlaced scan, alternating rows of pixels are refreshed in each cycle. This means that in a 60hz signal, alternating pixels rows are refreshed at 30hz each. Refreshing only half of the pixels per cycle reduces the bandwidth required for the display.
Bandwidth considerations like this were of greater concern for analog television and CRT, where bandwidth costs were high throughout the entire production. As the bandwidth in devices and Internet connections both increase, and people generally move toward higher image fidelity, progressive scan is generally preferable to interlaced display.
The main drawback of interlaced scanning is that images tend to flicker, and motion -- especially vertical motion -- appears jerky. LCD displays are natively progressive scan; displaying interlaced video requires conversion. Most LCD displays have converters, but for minimal quality loss, conversion requires ample visual processing power. Higher power graphics cards tend to include good quality de-interlacing as a feature. In some devices, de-interlacing leaves artifacts.