The phrase "non-avian dinosaurs" crops up in many articles. What does it mean? As we see below it means Dinosauria minus Aves. It lumps in the basal paraves (the primitive flying and secondarily flightless primitive birds) with the real dinosaurs such as Tyrannosaurus - all of them under the same label - "non-avian dinosaur". In other words, it lumps in basal paraves with actual dinosaurs.Why is this a problem?
The problem is that when someone makes a statement about "non-avian dinosaurs" we do not know whether they are referring to Paraves or whether they are referring to actual dinosaurs.
For example take a quote like the following:
"it was not until the early 1990s that clearly nonavian dinosaur fossils were discovered with preserved feathers. Today there are more than twenty genera of dinosaurs with fossil feathers, nearly all of which are theropods. "We cannot tell whether the quote refers just to feathered Paraves or whether it refers to actual dinosaurs. The way it is worded it leaves the impression that there were actually feathered dinosaurs.
So when someone claims that there are many feathered dinosaurs found, they are almost always referring to feathered Paraves which are not actually dinosaurs.
That is why the phrase "non-avian dinosaurs" (or "non-avian theropods") is not nearly precise enough. In fact, it is misleading.
Another example where they are talking only about oviraptors and not actual dinosaurs. But they call them "non-avian theropods".
Because Caudipteryx has clear and unambiguously pennaceous feathers, like modern birds, and because several cladistic analyses have consistently recovered it as a nonavian, oviraptorid, dinosaur, it provided, at the time of its description, the clearest and most succinct evidence that birds evolved from dinosaurs. Lawrence Witmer stated: “The presence of unambiguous feathers in an unambiguously nonavian theropod has the rhetorical impact of an atomic bomb, rendering any doubt about the theropod relationships of birds ludicrous.”
On the other hand:
When someone does explicitly claim that an actual dinosaur had feathers, it turns out they are not feathers but bristles. (That is a separate subject).