Hydro is a term that can refer to many different things. In the context of a crossword, it can refer to a SPA, which is a facility that offers treatments such as massages, facials, and other forms of relaxation. Hydrotherapy is also a type of SPA treatment that uses water to help relax the body and reduce stress. Hydroelectricity is another use of the term, which is the production of electricity from the energy of moving water. Finally, hydrocarbon is a compound consisting of hydrogen and carbon atoms, which is used in the production of fuels and other products.