A powerup in New Super Mario Bros. U Deluxe that transforms Toadette into Peachette, a Princess Peach lookalike retaining some original form attributes, with Peach's abilities and more. Canonically, the game's official website states only she can use it.
Fans, however, used it to similarly transform Bowser, and from there on King Boo, then many more characters, and even personify items, from the Mario universe. More specifically in human female, but no additional abilities. The more esoteric the subject, the less “lookalike” or even “human” applies.
The trend is known as “peachifying” (ピーチ姫化) in Japan. The result is typically named with either an "-ette" suffix (in reference to Peachette), or by default a “princess” title like Peach ("-Hime" suffix in Japanese).
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According to Danbooru topic #15603, peachifying is its own distinct change and resulting character:
