Depending on the purpose of your paper and on your readers knowledge of the topic, you may want to give an extended definition of a key term or concept. In the following paragraph, Stephen Hawking defines scientific theory by first defining theory, then defining more specifically a "good" theory, and finally clarifying the definition with an example of a theory (Aristotle's) that does not fit and an example of one (Newton's) that does.
In order to talk about the nature of the universe and to discuss questions such as whether it has a beginning or an end, you have to be clear about what a scientific theory is. I shall take the simple-minded view that a theory is just a model of the universe, or a restricted part of it, and a set of rules that relate quantities in the model to observations that we make. It exists only in our minds and does not have any other reality (whatever that might mean). A theory is a good theory if it satisfies two requirements: It must accurately describe a large class of observations on the basis of a model that contains only a few arbitrary elements, and it must make definite predictions about the results of future observations. For example, Aristotle's theory that everything was made out of the four elements, earth, air, fire, and water, was simple enough to qualify, but it did not make any definite predictions. On the other hand, Newton's theory of gravity was based on an even simpler model, in which bodies attracted each other with a force that was proportional to a quantity called their mass and inversely proportional to the square of the distance between them. Yet it predicates the motions of the sun, the moon, and the planets to a high degree of accuracy.
—Stephen Hawking, "The Nature of Theories"
