Despite what you may have been told at school or college, it is almost never a good idea to begin a university essay with a definition taken from a dictionary. Dictionary definitions are of little use in constructing an argument, for much the same reason that thesauruses are of no help in improving your writing style: all they do is give you an approximation to a word's meaning, which can only be fully known from seeing it in context. This is why the big Oxford English Dictionary reprints the passages on which its definitions are based. If you must include a dictionary definition as part of your essay, it should be taken from the Oxford English Dictionary, as this is the only authoritative source.
When a question asks you to "define" something like Romanticism or the imagination, you are not meeting its expectations by simply copying out a dictionary definition without further discussion. To "define" a topic in this context means to identify key issues and relate them to the academic field so a question like this is giving you some guidance about the material which should be in the essay's introduction.