A vessel is a hollow object made to hold something. A cup is a vessel. A jar is a vessel. A bottle, a bowl, a barrel, a tank, a ship’s hull — all vessels.

A vessel has an inside separated from an outside by a wall. The wall gives the inside its shape. A cup is open at the top. A bottle narrows to a neck. A barrel is closed all around except for a bung. The shape of the inside determines what the vessel can hold and how you get things in and out.

The vessel is not what it holds. A cup of water is a cup and some water, not a new object. Pour the water out: still a cup. Pour in milk: same cup, different contents. This is the basic fact about vessels — they persist through changes in what they hold. You wash a jar and reuse it. You empty a ship and reload it. The vessel is the holding structure; the contents are the held thing.

The vessel is not its wall material either. A clay cup and a glass cup do the same job. You can describe “cup” without saying what it’s made of. In practice, the material matters — glass breaks, clay chips, steel rusts — but the concept “vessel” is about the shape and the holding, not the substance.

Vessels nest. A bottle goes in a crate. The crate goes in a shipping container. The container goes on a ship. At each level, the thing inside is contents and the thing outside is vessel. What counts as vessel and what counts as contents depends on which level you’re looking at.

The word comes from Old French vaissel, from Latin vascellum, diminutive of vas (“container, dish”). English keeps both the concrete sense (a drinking vessel, a blood vessel) and the extended sense (a ship, a vehicle for carrying). The extension is straightforward: a ship is a large vessel that carries cargo across water. The metaphor works because the structure is the same — a hollow thing, an inside, contents that come and go while the vessel stays.

Aristotle called the vessel “transportable place” (Physics IV.4). Place is the boundary around something that stays put. A vessel is the boundary around something that moves. That one-line distinction covers most of what there is to say about vessels as a category.