1) Synapsid 2) Synapsid reptile 3) Transitional fossil 
Dimetrodon (n; meaning `two measures of teeth`) is an extinct genus of synapsid that lived during the Early Permian, around 295–272 million years ago (Ma). It is a member of the family Sphenacodontidae. The most prominent feature of Dimetrodon is the large sail on its back formed by elongated spines extending from the vertebrae. It walked on f.....
extinct relative of primitive mammals that lived from about 286 million to 270 million years ago during the Early Permian Period, fossils of which ... [1 related articles]
(pronounced die-MET-roh-don) Dimetrodon was a prehistoric animal with a large sail; it was not a dinosaur but a pelycosaur , an early synapsid. 
[n] - carnivorous dinosaur of the Permian in North America having a crest or dorsal sail
noun carnivorous dinosaur of the Permian in North America having a crest or dorsal sail
a carnivorous dinosaur, of the genus Dimetrodon, dominant in North America during the Permian Period, up to 10 ft. (3.1 m) long and usually bearing spinal sails.