Lines of resolution is a measure of horizontal resolution in a video system, as measured by the number of vertical black lines that can be resolved on a white background. The term was invented in the early days of television

Television

Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...

. Since analog transmission of video is scan line

Scan line

A scan line or scanline is one line, or row, in a raster scanning pattern, such as a line of video on a cathode ray tube display of a television set or computer monitor....

-based, the same number of horizontal lines is always transmitted. However, several factors impede the ability to display fine detail within a line:

1. The camera or other source of material.
2. The storage and processing of the picture.
3. The transmission of the TV signal e.g. broadcast by radio
   Radio
   Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
   
    or by cable.
4. The reception and reproduction of the picture on a TV set.

The actual measure of 'lines of resolution' is how many vertical black-white lines can be resolved on a display. In comparison to modern ideas of resolution, 'lines of resolution' is an analog measurement that varies with the quality of the signal. i.e. 640x480 has 480 lines of resolution in the ideal cases, however a 640x480 image recorded to VHS would only have 250 lines, due to the bandwidth limitation imposed by the limited bandwidth of the tape.