Aa, a river in the province of North Brabant, Holland, flowing N.W. past Bois-le-Duc into the Meuse. The Duke of York was defeated on its banks by the French (Sept. 15, 1794). A dam at Bois-le-Duc prevents the Rhine making another exit into the North Sea. The name is allied to the Latin aqua (water), and consequently is applied to many other small rivers in North Europe.