ˈday-flower A flower that opens by day; spec. in U.S. the genus Commelyna or Spiderwort. 1688R. Holme Armoury ii. 99/2 The Virginian Spiderwort..may be called the Day Flower, for it opens in the day, and closes in the night.1866Treas.Bot., Day-flower, an American name for Commelyna.