Synapsids (Synapsida) (meaning "fused arch") (synonymous with theropsids (meaning "beast-face")) are the mammals and everything more closely related to mammals than to reptiles and birds. The synapsids include mammals, other therapsids and "pelycosaurs" like Dimetrodon and Edaphosaurus.
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.