d

depth: The depth of a window or pixmap is number of bits per pixel it has. The depth of a graphics context is the depth of the drawables it can be used in conjunction with for graphics output.
descendant: If W is an inferior of A, then W is a descendant of A.
device: Keyboards, mice, tablets, track-balls, button boxes, and so forth, are all collectively known as input devices. The core protocol only deals with two devices: the keyboard and the pointer.
direct color: A class of colormap in which a pixel value is decomposed into three separate subfields for indexing. One subfield indexes an array to produce red intensity values, the second subfield indexes a second array to produce blue intensity values, and the third subfield indexes a third array to produce green intensity values. The RGB values can be changed dynamically.
dispatching an event: The process of finding the appropriate contact and its actions.
double-click: A sequence of two clicks of the same button in rapid succession.
drawable: Both windows and pixmaps can be used as sources and destinations in graphics operations. These are collectively known as drawables. However, an :input-only window cannot be used as a source or destination in a graphics operation.