Stands for "High Definition." HD describes any video with a resolution greater than standard definition (SD) video. While "high-definition" has a broad scope, it can also refer to a specific HD resolution, such as 1920x1080 pixels.
Before HDTV was introduced in the late 1990s, the most common video resolutions were 640x480 (NTSC) and 720x576 (PAL and SECAM). The NTSC (National Television Standards Committee) format was standard in the U.S. and Japan, while Europe and other parts of the world used PAL (Phase Alternating Line) or SECAM (Sequential Color and Memory). These resolutions became known as "standard definition" (SD) only after "high definition" (HD) was introduced.
When HD devices first hit the market, there were three standard versions:
720p, also called "HD ready," was the standard DVD video format, while 1080i and 1080p, called "Full HD," were most often used HD broadcasts. Since early HD televisions only supported 1080i, the standard HD broadcast format was 1080i. It was not until Blu-ray discs became available that 1080p became standard.
Since "HD" refers to any resolution greater than standard definition, it also encompasses higher resolutions like 4K and 8K. However, to avoid ambiguity, 4K resolution (3840x2160) is typically called Ultra HD or UHD, while HD most often describes 1920x1080 video.