When discussing the essential motif of a piece of work, it's not uncommon to use the term "main theme." However, there are other synonyms that can be used to convey the same idea. For instance, the "primary idea" or "core concept" signify the overall focus of the piece. Another option might be to refer to the "central message" or "crux" of the work. The "prevailing notion" or "dominant subject" are also viable options to express the main theme. Regardless of which synonym is employed, all of these terms can describe the central component of a work and help to illuminate its meaning.