A water jet is a kind of fountain which shoots water from a small opening in the ground or at the surface of a body of water. Unlike a freestanding fountain, there is almost no visible structure when the jet is turned off, so that the structure itself is not ornamental but only the water coming from it. This form of structure may encompass multiple parts detached from each other: any group of related water jets in close proximity forms a single structure.

The visible display comes principally from the projected water. When the flow stops, little constructed ornament remains visible at the outlet.

Several related outlets close together can form one display. A freestanding fountain instead has a visible ornamental structure as well as its water.