a conceptual abstraction that allows us to describe complex color-mapping arrangements in clear, simple language
a X server resource, containing representation of color coding scheme, color bit depth, and modificability of the palette
A visual describes a way of interpreting a pixel value. The visual class and the pixel size attribute collectively describe a visual.
The specifications for color handling for a drawable, including visual class, depth, RGB/pixel, etc., are collectively referred to as a visual and - in GTK+ - are stored in a GdkVisual. The visual accounts for the differences between various types of hardware in determining the way pixel values are translated into visible colors. A screen may support only one type of visual. See also: Colormap, Drawable.
A visual describes how color information is stored in pixels. A screen may support multiple visuals. On modern hardware, the most common visuals are truecolor visuals, which store a fixed number of bits (typically 8) for the red, green and blue components of a color. On ancient hardware, one may still meet indexed visuals, which store color information as an index into a color map, or even monochrome visuals.
(n.) In the X protocol, the specifications for color handling for a drawable image, including visual class, depth, and type. The visual accounts for the differences between various types of hardware in determining the way pixel values are translated into visible colors within a window. A particular screen can support only one visual type.