When making a hypothesis（假说）or other statement， scientists must make sure that they will beunderstood by other researchers. Misunderstandings occur when there are different concepts of what is being discussed.
A definition answers the question， “What is it?” Sometimes a definition is necessary because a word or concept has more than one meaning. For example， whether carbon is a metal or nonmetal depends on how you define carbon. At other times， a definition is required because a term is being used in a special way. For example， physicists use the terms work and energy in ways that are more specific than their common meanings. A definition should be complete enough to include all the items in the category yet narrow enough to eliminate items that do not belong. The Greek philosopher Plato once defined man as a two-legged creature that has no feathers. The problem with Plato's definition was that it did not distinguish a man from other two-legged creatures without feathers. Communication between researchers is dependent on precise definitions of substances， concepts， processes， and ideas.......