Water heating is a thermodynamic process using an energy source to heat water above its initial temperature. Typical domestic uses of hot water are for cooking, cleaning, bathing, and space heating. In industry, both hot water and water heated to steam have many uses.

Domestically, water is traditionally heated in vessels known as water heaters, kettles, cauldrons, pots, or coppers. These metal vessels heat a batch of water, but do not produce a continual supply of heated water at a preset temperature. Rarely, hot water will be naturally occurring, usually from natural hot springs. The temperature will vary based on the consumption rate of hot water; the water becomes cooler as flow is increased.

Appliances for providing a more-or-less constant supply of hot water are variously known as water heaters, hot water heaters, hot water tanks, boilers, heat exchangers, calorifiers, or geysers depending on whether they are heating potable or non-potable water, in domestic or industrial use, their energy source, and in which part of the world they are found. In domestic installations, potable water heated for uses other than space heating is also called domestic hot water (DHW).