A group of extinct reptiles in the clade archosauria (which also includes their ancestors, thecodonts, their progeny, birds, and crocodilians).
For a long time, the term dinosauria ("terrible lizards") was considered informal, and at best suitable only for children's books and Ray Harryhausen movies. Now it is recognized that dinosauria is a legitimate classification –– a group of animals characterized by an elongate ilium and a perforate acetabulum.
The ilium is the top bone in the hip. In dinosaurs, this bone was elongated from front to back. The acetabulum is the hip socket. If you were to stick your finger into a human acetabulum, you would hit the back of the socket. But with dinosaurs, the acetabulum is "perforate." In other words, there is no back to it. You can stick your finger right through. However, I'm not so sure that your local dinosaur would be very happy if you did this.