To get a sense of what high definition (HD) really means, it helps to know something about resolution.
Getting to know the basics
Detail (amount of information in the picture) and sharpness (amount of brightness or contrast at the edges of objects) are both influenced by resolution. Specifications like this are very important to consider before purchasing. A video display's physical structure and electronics determine the maximum number of visible pixels it can deliver. This is defined as native resolution. Measured resolution indicates the performance derived from test patterns. While it may not be as high as the native resolution, it can never be higher.
In analog television, resolution was measured as lines of horizontal resolution or, more simply, TV lines (TVL). This is not to be confused with the number of horizontal scan lines. In other words, it is the number of visible vertical lines that can be counted horizontally, not across the entire screen, but across a horizontal distance no bigger than the picture height. If you visualize it correctly, you'll see that it's made up of black and white vertical lines.
How much resolution is enough?
The minimum resolution to achieve HD is 720 by 1280 pixels. Full HD is 1080 by 1920 pixels and Ultra HD (UHD) is either 2160 by 3840 (4K) or 4320 by 7680 (8K).
The most commonly used format is 1080p, also referred to as Full HD in a marketing context. Like UHD, there is limited 1080p broadcast programming but you can stream it, and most new HDTVs have 1080p signal processing for lower quality material. While 1080p is the standard resolution for Blu-ray, it is considered to be at a lower tier of quality in the streaming space.
Next in line is the 1080i format. Due to the fact that it provides the maximum number of scan lines in an interlaced format, it is what is used in most HDTV broadcasts that exist today. 4K (also known as Ultra HD) is the format that now dominates the high-end TV market. That being said, most of what you would end up watching on a 4KTV would be upscaled from lower resolutions, such as 1080p and 720p.
Finally, the 8K format shares the Ultra HD label with 4K, which can be confusing. While 8K provides twice as many horizontal and vertical pixels, 4K is the standard for your typical 16:9 aspect ratio. Programming is scarce, so nearly all of what you would end up watching on an 8KTV would be upscaled from lower-resolution sources, such as 4K and 1080p.
The information in this blog post was adopted from the 20th Anniversary Edition of the book, Practical Home Theater: A Guide to Video and Audio Systems (2022 Edition) by Mark Fleischmann.