A theoretical variable such as integrity, conscientiousness, or academic honesty may correspond to either a construct or a concept, but the standard idiom does not distinguish the two. One can describe the difference between constructs and concepts in terms of set theory. Constructs extend over actual cases, whereas concepts extend over both actual and possible cases. As such, theoretical claims made about, say, integrity as a construct differ from claims about integrity as a concept. The restriction of constructs to a specified population plays a central role in test validation and psychometric analyses aimed at distinguishing constructs from one another. The extension of concepts over possible populations plays a central role in the adoption of nonactual possibilities as goals in making efforts toward systemic change and also in the comparison of constructs across populations. The failure of the standard idiom, which conflates constructs with concepts, to provide a vocabulary that captures both population-dependent and population-independent aspects of variables recommends the modification of that idiom to distinguish constructs from concepts. This distinction suggests various changes in practice such as including the intended population in the names of constructs but not concepts. (Contains 1 table and 1 figure.)