What is a cephalium?
There are problems with previous attempts to define 'cephalium', such as via production of more hairs and spines, confluence of areoles, or periderm development at or underneath each areole after flowering. I propose using the term 'cephalium' only for a combination of these criteria, i.e. flowering parts of cacti that have confluent hairy or spiny areoles exterior to a thick periderm, where these hairs, spines, and periderms arise almost immediately below the shoot apical meristem, and with more hairs and spines on reproductive parts than on photosynthetic parts of the shoot. Periderm development and confluent areoles preclude photosynthesis of cephalia, which therefore lack or mostly lack stomata. There is almost always a discrete transition from photosynthetic vegetative tissues to a non-photosynthetic flower-bearing cephalium, both of which arise from the same shoot apical meristem. Cephalia have different phyllotaxy than vegetative parts of the shoot and appear to be on top of existing vegetative phyllotaxy. If flow-ering parts only have a subset of the above characteristics of cephalia, then I propose calling these structures 'pseudocephalia'.
Gorelick, R. (2016). What is a cephalium?. Bradleya (Vol. 34, pp. 100–124).