The phrase "hidden zip" refers to a zipper that is not easily visible or noticeable on a piece of clothing or accessory. It is designed to blend in with the fabric, making it less noticeable and giving a smoother appearance. Full definition
This skirt has a back hidden zip closure and wide waistband. (diaryofachicmommy.com)
A mid rise waist with belt loops has gold stamped button and hidden zip fly, with silver accents and five pocket detail front to back. (herboutique.com)