If a machine is hydraulic, a liquid does the heavy pushing. The liquid — often a special oil, not always water — is forced through pipes. Liquid does not squash easily, so a small push on one side can lift a car on the other. Brakes, diggers, dentist chairs, and the jack that lifts a tyre often work this way. Though actually the old Greek word was about a water organ: a musical instrument that used water and pipes. Today the liquid is usually oil, so “it runs on water” is a little too tidy — some systems still use water, many do not. The word is technical but common on machines and in factory news. Learners almost always meet the adjective: a hydraulic lift, hydraulic brakes. Hydraulic is not electric, and it is not hydroelectric: those are different ways of using power.