niche product
A product designed to fill a small, often overlooked market segment; commonly used in business and marketing contexts.
This company succeeded with niche products.
It's a strategy of targeting niche products that large companies don't enter.
ニッチ商品 is a direct loanword equivalent, often used interchangeably, but 隙間商品 carries a slightly more domestic or traditional business nuance.
Compound of 隙間 (すきま, 'gap, crevice') and 商品 (しょうひん, 'product, merchandise'). The term metaphorically refers to a product that fills a market gap.