'To a great extent, science is about arriving at definitions. What is a man? What is a number? Questions such as these require substantial inquiry. But were science is supposed to be precise and measured; definitions can be frustratingly vague and variable. Here, Nature looks at some of the most difficult definitions in science. Some are stipulative definitions, created by scientists for their convenience, but on which the community has not found consensus. Popular though they are, not everyone agrees on what is meant by 'paradigm shift' or 'tipping point'. Essential definitions - those that get at the question of what makes a thing a thing - can be just as troublesome. What is race, or consciousness? And what does it matter if there is no agreed-on meaning? The good news is that for every troublesome term there are thousands used every day with no problems. Scientists are competent, if unconscious wielders of definition…'