Water Heater, as defined by the some low bidders.
Although it is proper to use the term "water heater," specify a "storage water heater" if you want the basic design as seen in homes. If you specify only a "water heater," you may get a surprise.
The low bidder may give you the "other" water heater. These are finned tube heat exchangers over a burner, in a box-shaped cabinet. Typical brands are Teledyne Lars and Lochinvar. They have a thermostat and a flow switch, and the burner will not turn on unless there is a safe minimum flow that activates the flow switch. The water goes through the heat exchanger once, picks up 5 to 10 degrees heat and it's out.
These are not appropriate for most applications.
Suddenly you find that you have to buy expensive add-ons to make the water heater do what you expected, what a "storage water heater" does without all the add-ons.
If you want the water to heat up more than 5 to 10 degrees, you must recirculate the water through the water heater. That means you have to add a pump (with its requirements for space, support, electrical power and controlling it) and piping (insulated). And if you want to draw water off the system at a predictable higher temperature, you'll need to store some water at that higher temperature, which means you need a water storage tank.
These water heaters are great for swimming pools and spas, where you already have a pump and water storage (the pool or spa). They're probably not what you want for regular domestic hot water. I got stuck with one on a project, in the end we put it in surplus and bought a storage water heater that would do the job without all the complication.

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