Capitulum: An inflorescence of sessile flowers borne on a flattened and expanded portion of the inflorescence axis; the inflorescence of the sunflower family (Asteraceae).*
All the fall asters (Symphyotrichum spp.) produce many capitula when they bloom. Each capitulum, or flower head, is composed of many smaller flowers called florets.
*Pell, S.K., Angell, B. (2016). A botanist’s vocabulary: 1300 terms explained and illustrated. Portland: Timber Press.