Televisions use lines of resolution to create images. Older conventional models used 480 lines stacked on top of each other, but showed half the lines at a time, switching 30 times a second. This process is known as interlacing.
The first step towards what we call high definition today was progressive scanning, a display method where all 480 lines are displayed 6o times a second, resulting in a sharper picture.
Today, “high definition” describes one of three resolution types. Each label describes the number of lines used in the display and the method used to display them - 720p (progressive), 1080i (interlaced), and 1080p (progressive).