The electrochemical window (EW) of a substance is the electrode electric potential range between which the substance is neither oxidized nor reduced.  The EW is one of the most important characteristics to be identified for solvents and electrolytes used in electrochemical applications. The EW is a term that is commonly used to indicate the potential range and the potential difference. It is calculated by subtracting the reduction potential (cathodic limit) from the oxidation potential (anodic limit).
When the substance of interest is water, it is often referred to as the water window.
This range is important for the efficiency of an electrode.  Out of this range, the electrodes will react with the electrolyte, instead of driving the electrochemical reaction.  
In principle, ammonia has an extremely small electrochemical window, but thermodynamically-favored reactions less than 1 V outside the window are very slow.  Consequently, the electrochemical window for many practical reactions is much larger, comparable to water.  Ionic liquids famously have a very large electrochemical window, about 4&ndash;5 V.