cosmos. 1  the universe considered as a harmonious and orderly system. 2  Rare harmony; order. 3  any complete and orderly system. 4  pl. cosâ²mos any of a genus (Cosmos) of tropical American plants of the composite family, with featherlike leaves and heads of white, pink, or purple flowers. pl. cosÂ·mosÂ·es or cosmos Any of various mostly Mexican herbs of the genus Cosmos in the composite family, having radiate flower heads of variously colored flowers and opposite pinnate leaves, especially C. bipinnatus and C. sulphureus, widely cultivated as garden annuals.