Monad from the Greek word monos or μονάς (from the word μόνος, which means "one", "single", or "unique"), was a symbol referred by the Greek philosophers as "The First", "The Seed", "The Essence", "The Builder", and "The Foundation."  They also called it "Unity."  Source:  "Divine Proportion" by Priya Hemenway, page 51.
Own work
jossi (original)   and  bdesham