What, then, is a genius? More specifically, what is a creative genius? For once, I don't find a dictionary definition particularly helpful since it addresses none of the practical concerns: consensus, consistency, and production.
Suppose someone creates a work that a few people consider a masterpiece. By "few" I mean a tiny minority, even among fans of that particular subject and genre. Because common understanding is the purpose of language, we cannot identify the author as a genius because too many people disagree with the assignation. At most, we can preface the term with a disclaimer such as "In my opinion [this person is a genius]."
Suppose a person creates one brilliancy in a career besmirched by enumerable embarrassing failures. That artist may be a financial success but "genius"? No. There is simply too much evidence to the contrary. This is equally true of the individual who recreates the same item, perhaps with minor variations, over and over again.
Now suppose someone writes one gem and then quits. "Genius"? Hard to say, since genius comes in a pattern. Is the artist a one-shot-wonder? A flash in the pan? Consistency implies a sizeable number of confirming examples, notwithstanding the occasional clunker.
When, then, can we use the word without qualification?
The good news is that, contrary to what passes for conventional "wisdom" today, the products of genius are never difficult to spot. For example, take The Learning Channel's "Something Borrowed, Something New". Resident genius, Kelly Nishimoto, reworks dresses worn by the bride's mother decades earlier, presenting each as a palatable option to modern gowns. She does this every episode. The results amaze even those of us with no interest in fashion and design.
Watch a few of her shows. It may be a life-changing experience to be able to point at something or someone and say without equivocation or reservation "Now that is genius!"