eu-

Well; easily.

Greek eu, well, from eus, good.

Examples derived directly from Greek words include euphony, the quality of being pleasing to the ear (Greek euphōnia, based on phōnē, sound)—the valved brass musical instrument called the euphonium derives from the same root; eulogy, a speech or piece of writing that praises someone or something highly (Greek eulogia, praise); euphoria, a feeling or state of intense excitement and happiness (Greek euphoros, borne well, healthy, from pherein, to bear).

Examples created in English on Greek roots include euthanasia, the painless killing of a patient suffering from an incurable and painful disease (Greek thanatos, death, so literally ‘easy death’); eutrophication, excessive richness of nutrients in a body of water (Greek trephein, nourish); eukaryote, an organism whose cells contain a nucleus (Greek karuon, kernel); eubacterium, the ‘true’ bacteria and cyanobacteria, as distinct from archaea.

The opposite is dys‑. See also eury‑.