For instance, biologists these days like to say that the word "dinosaur" is inclusive of modern birds, since birds are descended from dinosaurs. This is consistant with biologists' tendency to categorize things monophyletically -- that is, if it is descended from something, it "is" that something. A bird is descended from a dinosaur, so it must be a dinosaur.
Linguists (such as those employed by dictionaries e.g. Webster and Oxford) tend to be more pragmatic -- concerned more about communication and usefulness rather than strictly adhering to monophyletic categories. They consider "dinosaur" to only refer to those animals that lived in the mesozoic period....that is, they explicitly exclude modern birds from the definition, because when most people use the term "dinosaur," they are not talking about birds.
This is similar to the way we exclude tetrapods (amphibians, reptiles and mammals) from the definition of "fish", even though tetrapods are descended from fish. That is, "fish" is, by definition, a paraphyletic category. Other examples are "monkey" and "ape", which, by definition, are paraphyletic and therefore exclude humans.
So who is the authority on the definition of these words? And if biologists are, isn't it being inconsistant to define the word "fish" to exclude a subset of its decendants, while refusing to do the same with the word "dinosaur"?