In fiber optics, a band of wavelengths at which an optical fiber is sufficiently transparent for practical use in communications applications. Synonyms spectral window, transmission window. 2. In imagery, a portion of a display surface in which display images pertaining to a particular application can be presented, usually by using a graphical user interface (GUI. ) Note: Different applications can be displayed simultaneously in different windows. 3. A period during which an event can occur, can be expected to occur, or is allowed to occur.