The term "construct" means:
In a scientific theory, particularly within psychology, a hypothetical construct is an explanatory variable which is not directly observable. For example, the concepts of intelligence and motivation are used to explain phenomena in psychology, but neither is directly observable. (Source)
The earliest reference I can find to the term "construct" is from Clark L. Hull (1943) pp. 21—22
In some cases there may be employed in scientific theory a whole series of hypothetical unobserved entities; such a series is presented by the hierarchy of postulated physical entities: molecule, atom, and electron,the molecule supposedly, being constituted of atoms and the atom in its turn being constituted of electrons.
A rough parallel to this chain of hypothetical entities from the physical sciences will be encountered in the present system of behavior theory. For the above reasons the subject of symbolic constructs, intervening variables, or hypothetical entities which are not directly observable requires comment.
Despite the great value of logical constructs or intervening variables in scientific theory, their use is attended with certain difficulties and even hazards.
Hull, C. L. Principles of behavior. New York: Appleton-Century, 1943 [Free PDF]