Archosauria - "ruling lizards"
A clade of animals that includes dinosaurs, birds, pterosaurs (the winged reptiles that co-existed with dinosaurs), crocodilians, and thecodonts (the group of reptiles that eventually gave rise to dinosaurs).
The most recognizable characteristic of an archosaur is the presence of antorbital fenestras: openings in the skull, just ahead of the eyes.
The following excerpt is from The Evolution and Extinction of the Dinosaurs by David E. Fastovsky and David B. Weishampel, Chapter 4, p. 87
Archosauria
Finally we come to the other great clade of diapsids, Archosauromorpha (archo - ruling; sauros - lizard; morpho - shape). Of particular interest to us, however, is the famous clade within the archosauromorphs, Archosauria (archo - ruling; sauros - lizard) among whom are the crocodiles, dinosaurs, pterosaurs, and birds.
Archosauromorpha is supported by many important, shared, derived characters, which are included on the cladogram in Figure 4.15. Within the archosauromorphs are a series of basal members that are known mostly from the Triassic. Some bear a superficial resemblance to large lizards (remember, however, that they cannot be true lizards, which are lepidosaurs), wheras others look like reptilian pigs.
The last of the aforementioned - prolacertiforms - possess a number of significant evolutionary innovations, most notably an opening on the side of the snout, just ahead of the eye, called the antorbital fenestra. (...)
Crocodilians and their close relatives belong to a clade called Crurotarsi (cruro - shank; tarsus - ankle); birds and their close relatives constitute a clade called Ornithodira (ornith - bird; dira - neck). Crocodiles have had a glorious and diverse history, and in the past, there have been seagoing crocodiles with flippers instead of legs, crocodiles that appear to have been well adapted for running on land. Other crurotarsans included a variety of carnivorous, piscivorous (fish-eating), and herbivorous forms.
Ornithodira brings us quite close to the ancestry of dinosaurs. This group is composed of two major clades, Dinosauria (deinos - terrible; sauros - lizard) and Pterosauria (ptero - winged; sauros - lizard). Ornithodirans are united by the fact that within the archosaurs, they alone possess an erect, or parasagittal, posture -- that is, a posture in which the plane of the legs is perpendicular to the plane of the torso and is tucked under the body.