Cheekini: A style of women’s underwear with moderate coverage in front and on the sides and a raised cut in the rear that covers some but not all of the buttock cheeks.There's more information about cheekinis at Fritinancy, which also offers this evocative alternative term for thongs: "butt floss."
The style sometimes appears, with minor variations, under different names: boykini panties, boyleg briefs, hipster panties, booty shorts. All are marketed as comfortable, panty-line-less alternatives to thong underwear.
I first discovered cheekini on the Victoria’s Secret website, but the term is neither original nor exclusive to that retailer. JC Penney sells cheekini panties under its Flirtitude label; it also sells visually similar boykini panties and hipster panties.
Image from Victoria's Secret.