The amount that such a bottle holds, about 1.13 litres.
A large vessel usually with a handle, spout and lid, for drinks such as wine or cider.
2003, Fran Walsh, Philippa Boyens, & Peter Jackson, (w), 00:14:12:
Merry and Pippin: You can drink your fancy ales / You can drink 'em by the flagon / But the only brew for the brave and true / Comes from The Green Dragon