The prefix hydro- is from the Greek prefix hudro- or hudr-, which comes from the Greek noun hudor, meaning “water.” Thus in the word hydrophobia, hudro- combines with the suffix -phobia, “fear,” to mean “an abnormal fear of water.” Hydrophobia is an example of a Greek word that was later adopted into Latin, then French, and then English. Hydroelectric, hydroplane, and hydrosphere are examples of words more recently formed in English. Before a vowel, hydro- sometimes becomes hydr-: hydrate, hydrous.
