Dot pitch

Dot pitch is a specification for a computer display, computer printer, image scanner, or other pixel-based devices that describe the distance, for example, between dots on a display screen. In the case of an RGB color display, the derived unit of pixel pitch is a measure of the size of a triad plus the distance between triads.
Dot pitch may be measured in linear units, usually millimeters, or as a rate, for example, dots per inch. Closer spacing produces a sharper image. However, other factors may affect image quality, including:

Undocumented or inadequately documented measurement method, complicated by ignorance of the existence of different methods
Confusion of pixels and subpixels
Element spacing varying across screen area
Differing pixel geometries
Differing image and pixel aspect ratios
Miscellanea such as Kell factor or interlaced video

The exact difference between horizontal and diagonal dot pitch varies with the design of the monitor, but a typical entry-level 0.28 mm monitor has a horizontal pitch of 0.24 or 0.25 mm, and a good quality 0.26 mm unit has a horizontal pitch of 0.22 mm.
The above dot pitch measurement does not apply to aperture grille displays. Such monitors use continuous vertical phosphor bands on the screen, so the vertical distance between scan lines is limited only by the video input signal's vertical resolution and the thickness of the electron beam, so there is no vertical 'dot pitch' on such devices. Aperture grille only has horizontal 'dot pitch', or otherwise known as 'stripe pitch'.