Bay windows are folds or bulges in a building's facade, extending up from the ground or base of the building (or at least from the base of that facade), usually across several floor levels, which add a small nook to the interior space and provide side-facing windows for additional light and views. Many buildings feature parallel bay windows, but they can also exist singly.

A bay window is similar to an oriel, except that a bay window reaches all the way down to the ground or base, whereas an oriel is cantilevered over outdoor space.

The projecting facade reaches its base and creates an interior nook with side-facing windows. The number of repeated bays does not determine the feature: one bay is enough, while the unsupported space beneath an oriel provides the distinction between the two forms.