Define means “to set bounds to.” You define a thing, word, or concept by describing it to distinguish it from all similar things. If people don’t agree on the meaning of a word or an idea, they can’t share knowledge about it. Scientists take special care to define their terms precisely. In “A Chemist’s Definition of pH” from The Condensed Chemical Dictionary (New York: Reinhold, 1981), Gessner G. Hawley begins with a brief definition:

pH is a value taken to represent the acidity or alkalinity of an aqueous solution; it is defined as the logarithm of the reciprocal of the hydrogen-ion concentration of a solution:

If you use a word in a special sense or invent a word, you have to explain it or your readers will be lost. In “The Futile Pursuit of Happiness” (New York Times 7 Sept. 2003), Jon Gertner reports on “affective forecasting,” an intriguing area of study by economists and psychologists such as Professors Daniel Gilbert and Tim Wilson. They are exploring what people expect will bring them happiness and how their expectations pan out. Not surprisingly, their new area of study has generated new terms.

Gilbert and his collaborator Tim Wilson call the gap between what we predict and what we ultimately experience the impact bias — impact meaning the errors we make in estimating both the intensity and duration of our emotions and bias our tendency to err. The phrase characterizes how we experience the dimming excitement over not just a BMW but also over any object or event that we presume will make us happy. Would a 20 percent raise or winning the lottery result in a contented life? You may predict it will, but almost surely it won’t turn out that way. And a new plasma television? You may have high hopes, but the impact bias suggests that it will almost certainly be less cool, and in a shorter time, than you imagine. Worse, Gilbert has noted that these mistakes of expectation can lead directly to mistakes in choosing what we think will give us pleasure. He calls this miswanting.

You might define an unfamiliar word to save readers a trip to the dictionary or a familiar but often misunderstood concept — such as guerrilla, liberal, or minimum wage — to clarify the meaning you intend. The more complex or ambiguous the idea, thing, movement, phenomenon, or organization, the longer the definition you will need to clarify the term for your readers.