Definition of window
1a : an opening especially in the wall of a building for admission of light and air that is usually closed by casements or sashes containing transparent material (such as glass) and capable of being opened and shut
b : windowpane
c : a space behind a window of a retail store containing displayed merchandise
d : an opening in a partition or wall through which business is conducted a bank teller's window
2 : a means of entrance or access especially : a means of obtaining information a window on history