Fire hydrant (also known as fire hydrant, fire pump, Johnny pump or pump for short) is the connection point through which firefighters can enter the water supply system. It is an integral part of active fire protection.

In areas subject to freezing temperatures, at most only a portion of the hydrants are above the ground. The valve is located below the frost line and is connected to the above ground part through a standpipe. The stem extends upward from the valve through the seal at the top of the hydrant, where it can be operated with a suitable wrench. This design is called a "dry barrel" hydrant because the vertical body of the barrel or hydrant is usually dry. When the water valve is fully closed, the underground drain valve opens. This will allow all water to drain from the hydrant to prevent the hydrant from freezing.

In warm areas, fire hydrants and one or more valves are used above ground. Unlike cold hydrants, the water supply to each port can be turned on and off. This style is called "wet barrel" hydrant.

Wet and dry barrel hydrants usually have multiple outlets. The outlet of the wet barrel fire hydrant is usually controlled separately, while a single stem operates all the outlets of the dry barrel fire hydrant at the same time. As a result, wet barrel hydrants allow more effort to open a single outlet, but also allow greater flexibility.

A typical American dry barrel hydrant has two smaller outlets and one larger outlet. If the hose has been standardized by the local fire department with Storz fittings for large diameter water supply pipes, the smaller outlet is usually a Storz connection. Larger outlets are called "steam pot" connections because they were once used to supply water to steam driven pumps, and hydrants with such outlets may be called "steam pot hydrants," although the use has become obsolete. Similarly, the old fire hydrant without steam connection can also be called "rural fire hydrant".