I would only define one word (or two words MAXIMUM) - the way to choose this word to define is to think about what the underlying concept is of the question. e.g. if there is a question about trade and financial flows, I would be defining GLOBALISATION, rather than trade and financial flows, as globalisation is what underlies trade and financial flows and really binds together the whole essay. don't get carried away defining heaps and heaps of words - you need to remember that when writing an economics essay, the focus is on ANALYSIS not description.