High-definition, more frequently referred to as HD, refers to a level of detail on screen measured in terms of pixels. A pixel is the smallest visible element on a display, which are the miniature dots that make up the picture. Any HD television set or computer monitor has at least 720 pixels, if not 1,080 pixels. These higher pixel counts provide more fluid video and more vibrant colours than other pixel counts, such as 240, 360 or 480.
High-definition technology was developed throughout the 20th century and was introduced in the United States in the late 1980s. Its use has become increasingly prevalent in the 21st century, even as the technology continues to improve. As of 2016, most television stations offer both SD and HD channels so that viewers with HD-enabled television sets can watch their programs at the optimal level.
As of 2016, there are three HD resolutions. The first is 720p, which is also known as HD Ready. The second is 1080p, which is known as Full HD. The only difference between these first two resolutions is level of detail; 720p provides a picture that is approximately twice as vivid as standard definition, whereas 1080p provides a picture that is about five times as detailed.
The third resolution is 1080i. The "i" in 1080i stands for "interlaced scan." Interlaced scan, unlike the progressive scan used in 720p and 1080p, displays each odd line of the picture followed by each even line. The resulting image is not as smooth as a progressive feed, but 1080i tends to provide a more detailed picture that is best suited to documentaries and wildlife footage. However, 1080i is not as suitable for action-oriented material, including sports and movies.