Diapsid is a term in biology that refers to reptiles that have two openings in their skulls behind their eyes. However, there are several synonyms for diapsid that can also be used to describe these animals. One common synonym is "double-skulled reptiles," which emphasizes the two openings in their skull. Another synonym is "diapsidans" which is a broader term that includes animals such as dinosaurs, birds, and other reptiles. Some also refer to diapsids as "lizard-like" due to their scaly skin and reptilian appearance. Overall, these synonyms help to better understand the diversity of animals within the diapsid classification.