Webster's Revised Unabridged Dictionary · 1913

Oriel

Oriel , noun

[Old French oriol gallery, corridor, Late Latin oriolum portico, hall, prob. from Latin aureolus gilded, applied to an apartment decorated with gilding. See Oriole.]

1.
A gallery for minstrels. [Obsolete] — W. Hamper
2.
A small apartment next a hall, where certain persons were accustomed to dine; a sort of recess. [Obsolete] — Cowell
3.
(Architecture) A bay window. See Bay window.
The beams that thro' the oriel shine Make prisms in every carven glass. — Tennyson

There is no generally admitted difference between a bay window and an oriel. In the United States the latter name is often applied to bay windows which are small, and either polygonal or round; also, to such as are corbeled out from the wall instead of resting on the ground.