plural noun
Syn·​ap·​si·​da | \ -psədə \

Definition of Synapsida

: a subclass of Reptilia comprising extinct reptiles of the Pennsylvanian, Permian, and Triassic, having a single pair of lateral temporal openings in the skull, and usually held to be ancestral to the true mammals — compare ictidosauria , therapsida

Love words?

You must — there are over 200,000 words in our free online dictionary, but you are looking for one that’s only in the Merriam-Webster Unabridged Dictionary.

Start your free trial today and get unlimited access to America's largest dictionary, with:

  • More than 250,000 words that aren't in our free dictionary
  • Expanded definitions, etymologies, and usage notes
  • Advanced search features
  • Ad free!

History and Etymology for Synapsida

New Latin, from synapsis + -ida

Keep scrolling for more

Learn More about Synapsida

Statistics for Synapsida

Cite this Entry

“Synapsida.” Dictionary, Merriam-Webster, Accessed 24 Sep. 2020.

Comments on Synapsida

What made you want to look up Synapsida? Please tell us where you read or heard it (including the quote, if possible).


Test Your Vocabulary

Homophone Quiz

  • three bears two of them look like theyre whispering to a third bear who looks chuffed to be the center of attention
  • In order to judge how people felt, the senator's office hired a firm to take a ______.
Spell It

Can you spell these 10 commonly misspelled words?

Add Diction

Test Your Knowledge - and learn some interesting things along the way.

Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!