Plant taxonomy classifies 'Emerald Green' Arborvitae under Thuja occidentalis. 'Emerald Green' arborvitae is one of its cultivars. The common name for these conifers is often misspelled, "arbor vitae" (technically, when spelled as two words, it's a part of the human anatomy, not a tree). The term is Latin and means "tree of life," due to the alleged medicinal value of its resin. Oddly, this is a case in which a plant's common name and scientific name, while different, are both Latin!