Belonging to the family Compositæ, in which what is popularly called the flower consists really of a close head of many small flowers (‘florets’) sessile on a common receptacle, and surrounded by a common involucre of bracts; examples are the daisy, dandelion, tansy, marigold, aster, chrysanthemum, dahlia, sunflower. Also n. A plant of this order. In many Composites the florets of the ray or circumference differ in shape from those of the disc, being developed so as to look like petals; by cultivation, the florets of the disc may assume the same form, as in the daisy, dahlia, and common marigold; these two states are popularly called single and double.
Source: Oxford English Dictionary