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 and 1080p video content.
In audio, HD Voice provides greater audio clarity than standard phone voice quality.
See: HD Voice
Some phones have "HD" microphones that are actually combination of two microphones: one for low volumes, and one for loud sound (also called high-pressure audio.) This allows them to capture sound of all volumes with good quality.