A soil-less grow medium that delivers nutrients through water as opposed to dirt. Hydro refers to hydroponic systems used in the process. This system is a plant-growing technique that does not require soil. All the plant's nutrients are delivered through nutrients in the water, as opposed to the conventional method of topsoil. The traditional topsoil-farming methods require lots of land and space, but a hydroponic system is much more efficient in terms of plant-to-space ratio. The term hydro can also be another synonym for cannabis, but only when the weed has been grown in a hydroponic system.