Synapsids are characterised by single hole (temporal fenestra) behind each eye orbit. The fenestra is located below the articulation of the post-orbital and squamosal bones of the skull. In later synapsids the fenestrae became larger and in mammals they are called zygomatic arch.
In evolution of therapsid synapsids the jaw-joint bones quadrate and articular became middle ear bones, "anvil" (incus) and "hammer" (malleus).
Synapsid other than mammals were once referred to as "mammal-like reptiles" but because they are not closely related to true reptiles this name is somewhat misleading.
"Pelycosaurs" are not a real group, instead they are the term used for synapsids that are not therapsids.