1 Oculus, or any circular element placed in the centre of something, e.g. a bull's-eye circular window in the middle of the tympanum of a pediment.
2 Circular or nearly circular central part of a volute, as in an Ionic capital.
3 Very small, more or less triangular, light in Gothic tracery.
4 Circular base or rim of a cupola, i.e. the circle from which the domed part springs.