Reptiles are air-breathing, cold-blooded vertebrates that have skin covered in scales as opposed to hair or feathers. They are tetrapods (having or having descended from vertebrates with four limbs) and amniotes, whose embryos are surrounded by an amniotic membrane.
By this definition, I think it is safe to point out that Dinosaurs don't quite fit in this category. Especially after it has been found that most of them were warm blooded and more and more discoveries support that more and more species of them have been discovered to have had feathers.
While I agree with the consensus to place most dinosaurs in the same class as birds, I'm not sure what to think about species of tetrapods such as Brachiosaurus and triceratops, and other dinosaurs utilizing all four of their legs for walking.