n.
      - an opening in a wall or roof, fitted with glass in a frame to admit light or air and allow people to see out. 
▸an opening through which customers are served in a bank, ticket office, etc.
 ▸a space behind the window of a shop where goods are displayed.
 ▸
(a window on/into/to)
 a means of observing and learning about.
 - a transparent panel in an envelope to show an address.
 - Computing  a framed area on a display screen for viewing information.
 - an interval or opportunity for action.
 - Physics  a range of electromagnetic wavelengths for which a medium (especially the atmosphere) is transparent.
 - strips of metal foil dispersed in the air to obstruct radar detection. 
[military code word.]
 
 
  Phrase
  
    - go out 
(of)
 the window
 informal (of a plan or behaviour) be abandoned or cease to exist.  - windows of the soul
 the eyes. 
   
  Derivative
  
    - windowed adj.
  - windowless adj.
 
   
  Etymology
  ME: from ON vindauga, from vindr ‘wind’ + auga ‘eye’.