" Defining words properly is a fine and peculiar craft. There are rules—a word (to take a noun as an example) must first be defined according to the class of things to which it belongs (mammal, quadruped), and then differentiated from other members of that class (bovine, female). There must be no words in the definition that are more complicated or less likely to be known that the word being defined. The definition must say what something is, and not what it is not. If there is a range of meanings of any one word—cow having a broad range of meanings, cower having essentially only one—then they must be stated. And all the words in the definition must be found elsewhere in the dictionary—a reader must never happen upon a word in the dictionary that he or she cannot discover elsewhere in it. If the definer contrives to follow all these rules, stirs into the mix an ever-pressing need for concision and elegance—and if he or she is true to the task, a proper definition will probably result. "

― Simon Winchester , The Professor and the Madman: A Tale of Murder, Insanity and the Making of the Oxford English Dictionary