The simple answer is that a niche perfume comes from a company or brand that only makes fragrance

The best niche perfumes feel like hidden gems with their air of exclusivity and cool—like you stumbled upon them at a chic indie perfume store or picked up a bottle on your Euro summer vacation. But first, what exactly is a niche perfume, anyway? The simple answer is that a niche perfume comes from a company or brand that only makes fragrance—not a designer brand that creates scents along with clothing, eyewear, and makeup, for example, or a makeup brand that also offers fragrance alongside lipstick and mascara, like Rare Beauty, Chanel, Tom Ford, and Glossier. Many niche fragrance brands—also called indie fragrance brands—put a lot of care and artistry into their formulations and view perfume as a true art, which is reflected in their scents. — Not my simple answer, mind you, but read more in The Best Niche Perfumes Will Make You Stand Out in a Crowd at Allure.