A broad term applied to any number of digital media formats and/or technologies that provide improved detail and resolution. It is used primarily with regard to video images, but is also sometimes used to refer to still images, and sometimes audio.
Audio/visual content can be HD. That content must be captured with HD-capable technology, and requires HD-capable technology to view in full detail.
Most commonly, HD refers to 720p and 1080p (or 1080i) video content.
It is also used to refer to phone displays with pixel dimensions that match 720p or 1080p video content. 720 x 1280 pixels is the minimum resolution considered "HD", while 1080 x 1920 pixels is often called "full HD" (FHD).
See: Full HD
Often a plus sign is added to indicate that the display technically has more pixels than 720 x 1280. For example, a modern phone with an "HD+" display might have 720 x 1560 pixels.
In this context, HD is also used as a baseline for multiples, including Quad-HD.
See: Quad HD
In audio, HD Voice provides greater audio clarity than standard phone voice quality.
See: HD Voice