When used as a specific technical term used to distinguish from standard definition television in the context of High Definition Television (HDTV) the term high definition has become associated with a particular set of technical specifications or with the values of certain parameters within such specifications.
These parameters which distinguish high definition are concerned with the resolution of displayed images.
In analogue (as opposed to digital) terms, the parameter of resolution is lines (the image is divided into a number of horizontal scan lines ) but in digital terms the image is described in terms of numbers of pixels of horizontal and vertical resolution.