D.1. Economics: It is the scientific study of that aspect of human behavior which results from the conflict between multiple ends, and scarce but substitutable multiple means.
D.2. Buying and Selling as Aspect of Behavior: Unlike a category, such as buying or selling, it is the aspect, or predicate, of any behavior subject to the condition of means and ends which economics studies.
E.1. In the same sense as, for instance, wealth is an aspect of, say, a small well in a large desert, but not in a lush green land with streams of water. Or weapons, during war, are a wealth but immediately, a few minutes after ceasefire, become an utter waste.
D.3. Ends: These are multiple motives which have significance for our actions. These are never singular.
D.4. Means: These are multiple resources at our disposal and do not allow for simultaneous gratification of all our ends.
D.5. Substitutable: The scarce Means must be such that they can be exchanged with each other, or postponed for later consumption.
D.6. Relative Valuations: These are the different ratios of valuation between substitutable means resulting from the different degrees of their scarcity which in turn depends on variations of means and ends.
D.7. Exchange Relations: These are the dealings in an economy, and they can be explained only in terms of the dynamics of behavior of each agent under the conditions of relative scarcity of means in relation to ends.
Reference: Chapter 01 of Robbins, L. (2007). An essay on the nature and significance of economic science.