What is the circular stained-glass window common in Gothic churches called?

The circular stained-glass window common in Gothic churches is called a rose window.

Rose windows are large round openings filled with stone tracery and coloured glass. Their radial designs often resemble the petals of a flower, which explains the name. In French architectural writing, the term roue, meaning wheel, also appears for related circular window forms.

The type developed from earlier Romanesque wheel windows and became especially prominent in Gothic architecture. Major examples appear on the façades and transepts of French cathedrals such as Notre-Dame de Paris and Chartres Cathedral. A rose window could provide strong symbolic imagery as well as daylight, often using geometric divisions to organize scenes or figures.

A rose window is not simply any round window. A circular opening without elaborate tracery may be called an oculus, while a lancet is a tall, narrow pointed window. The exact tracery pattern and date vary widely from one building to another.