A Cocotte is an ovenproof dish. It can be made of ceramic or enamelled cast iron.
It is usually thought of as ramekin size, for baking eggs in, as in “oeuf cocotte” which is shirred eggs. This size has no lids. But they can be large, 5-quart ones with lids, used on top the stove for braising and simmering, and popped in the oven for baking.
Some larger ones have lids that have spikes on the underside, to concentrate where the dripping happens so that the dish cooking inside becomes self-basting.
These larger ones, basically what we would call casserole dishes in English, are sometimes referred to as “French ovens.” They can be used on any heat source.