Subclass Archaeornithes is a biological term that refers to a specific classification of extinct avian dinosaur species. The term Archaeornithes itself is derived from Greek words that mean "ancient bird." There are a few different synonyms for this term, such as Enantiornithes, which is another subdivision of avian dinosaurs that lived during the Cretaceous period. Another synonym is Aves, which refers to modern-day birds and their closest extinct ancestors. However, it is important to note that while some Archaeornithes may have had features similar to modern birds, they were not true birds in the sense that they did not possess the same unique skeletal structure that birds have, such as keeled sternums for flight.