One example of an evening primrose.
evening primrose definition by Webster's New World
any of a genus (Oenothera) of plants of the evening-primrose family, with yellow, pink, or white flowers, many of which open in the evening
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
designating a family (Onagraceae, order Myrtales) of dicotyledonous plants found chiefly in temperate America, including the fuchsia, sundrops, and willow herbs
evening primrose definition by American Heritage Dictionary
Any of various North American plants of the genus Oenothera, characteristically having four-petaled yellow flowers that open in the evening. Also called sundrops.