The recurring thing that gives a body of work its unity. Also the coherent set of surfaces a system wears.
Greek θέμα (thema, that which is laid down, a proposition), from τίθημι (tithenai, to place); the recurring thing that gives a body of work its unity is exactly what the Greek word first named, the thing set down as the subject.
The recurring thing that gives a body of work its unity, or the coherent set of surfaces a system wears. Both name what has been laid down as the subject.