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 where 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'.
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