Science, Tech, Math › Science Definition of the Biology Prefix 'Eu-' Biology prefixes and suffixes help us to understand biology terms Share Flipboard Email Print Euglena are eukaryotic protists. Gerd Guenther/Science Photo Library/Getty Images Science Biology Basics Cell Biology Genetics Organisms Anatomy Physiology Botany Ecology Chemistry Physics Geology Astronomy Weather & Climate By Regina Bailey Biology Expert B.A., Biology, Emory University A.S., Nursing, Chattahoochee Technical College Regina Bailey is a board-certified registered nurse, science writer and educator. Her work has been featured in "Kaplan AP Biology" and "The Internet for Cellular and Molecular Biologists." our editorial process Regina Bailey Updated September 10, 2019 The prefix (eu-) means good, well, pleasant or true. It is derived from the Greek eu meaning well and eus meaning good. Examples Eubacteria (eu - bacteria) - kingdom in the bacteria domain. Bacteria are considered to be "true bacteria", distinguishing them from archaebacteria. Eucalyptus (eu - calyptus) - a genus of evergreen tree, commonly called gum trees, that are used for wood, oil, and gum. They are so named because their flowers are well (eu-) covered (calyptus) by a protective cap. Euchlorine (eu - chlorine) - an older, outdated chemistry based term that refers to a chlorine based gas which was composed of both chlorine and chlorine dioxide. Euchromatin (eu - chroma - tin) - a less compact form of chromatin found in the cell nucleus. Chromatin decondenses to allow DNA replication and transcription to occur. It is called true chromatin because it is the active region of the genome. Eudiometer (eu - dio - meter) - an instrument designed to test the "goodness" of air. It is used to measure gas volumes in chemical reactions. Eudiploid (eu -diploid) - refers to an organism that is both diploid and euploid. Euglena (eu - glena) - single-celled protists with a true nucleus (eukaryote) that have characteristics of both plant and animal cells. Euglobulin (eu - globulin) - a class of proteins known as true globulins because they are soluble in saline solutions but insoluble in water. Euglycemia (eu - gly - cemia) - a medical term that refers to a person who has a normal level of glucose in their bloodstream. Eukaryote (eu - kary - ote) - organism with cells containing a "true" membrane bound nucleus. Eukaryotic cells include animal cells, plant cells, fungi and protists. Eupepsia (eu - pepsia) - describes good digestion due to having the appropriate amount of pepsin (gastric enzyme) in gastric juice. Eupeptic (eu - peptic) - of or relating to good digestion based on having the correct amount of gastric enzymes. Euphenics (eu - phenics) - the practice of making physical or biological changes in order to address a genetic disorder. The term means "good appearance" and the technique involves making phenotypic changes that don't alter a person's genotype. Euphony (eu - phony) - agreeable sounds that are pleasing to the ear. Euphotic (eu - photic) - relating to the zone or layer of a body of water that is well lit and receives enough sunlight for photosynthesis to occur in plants. Euplasia (eu - plasia) - the normal condition or state of cells and tissues. Euploid (eu - ploid) - having the correct number of chromosomes that corresponds to an exact multiple of the haploid number in a species. Diploid cells in humans have 46 chromosomes, which is twice the number found in the haploid gametes. Eupnea (eu - pnea) - good or normal breathing that is sometimes referred to as quiet or unlabored breathing. Eurythermal (eu - ry - thermal) - having the ability to tolerate a wide range of environmental temperatures. Eurythmic (eu - rythmic) - having a harmonious or pleasing rhythm. Eustress (eu - stress) - a healthy or good level of stress that is considered beneficial. Euthanasia (eu - thanasia) - the practice of ending a life in order to alleviate suffering or pain. The word literally means a "good" death. Euthyroid (eu - thyroid) - the condition of having a well functioning thyroid gland. In contrast, having an overactive thyroid is known as hyperthyroidism and having an underactive thyroid is known as hypothyroidism. Both hyperthyroidism and hypothyroidism can lead to a number of serious health consequences. Eutrophic (eu - trophic) - a term typically used for a body of water such as a pond or lake that has a bevy of organic nutrients that can promote aquatic plant and algae growth. This growth can lead to a reduction in the level of oxygen in the body of water that can have adverse consequences for animals that live in the water. Eutrophy (eu - trophy) - the state of being healthy or having well balanced nutrition and development. Euvolemia (eu - vol - emia) - the state of having the proper amount of blood or fluid volume in the body.