The Council of Vienne (1311-1312), under the authority of Clement V, defined the dogma that the soul is the form of the human body (Denzinger 481):
Moreover, with the approval of the said council, we reject as erroneous and contrary to the truth of the catholic faith every doctrine or proposition rashly asserting that the substance of the rational or intellectual soul is not of itself and essentially the form of the human body, or casting doubt on this matter. In order that all may know the truth of the faith in its purity and all error may be excluded, we define that anyone who presumes henceforth to assert defend or hold stubbornly that the rational or intellectual soul is not the form of the human body of itself and essentially, is to be considered a heretic.
"Form" is "the actualizing principle that makes a thing to be what it is." Thus, a body without the substantial form of a human being (a soul) is a corpse.
The Oxford English Dictionary defines form as:
In the Scholastic philosophy: The essential determinant principle of a thing; that which makes anything (matter) a determinate species or kind of being; the essential creative quality.
This use of form (Aristotle's μορϕή or εἶδος) and matter (ὕλη) is a metaphorical extension of their popular use. In ordinary speech, a portion of matter, stuff, or material, becomes a ‘thing’ by virtue of having a particular ‘form’ or shape; by altering the form, the matter remaining unchanged, we make a new ‘thing’. This language, primarily applied only to objects of sense, was in philosophical use extended to objects of thought: every ‘thing’ or entity was viewed as consisting of two elements, its form by virtue of which it was different from, and its matter which it had in common with, others.
Read St. Thomas Aquina's short work On the Principles of Nature (De principiis naturæ) for more information on matter and form (hylemorphism).
St. Thomas Aquinas also addresses the question of the union of a human's intellectual soul with the human's body in Summa Theologica I q. 76.