A lancet window is a tall narrow window with a pointed arch at its top. It acquired the "lancet" name from its resemblance to a lance. Instances of this architectural motif are most often found in Gothic and ecclesiastical structures, where they are often placed singly or in pairs.
The motif first appeared in the French early-Gothic period (c. 1140-1200) and later in the English period of Gothic architecture (1200-1275). So common was the lancet window feature that this era is sometimes known as the "Lancet Period". Strictly speaking, the lancet window should be austere and without tracery. However, it is not uncommon to find the form sub-divided and filled with stained glass, especially when dating from the Gothic-revival period of the nineteenth century.