A foldable phone is a smartphone with a flexible display and a hinge, letting it fold from a pocketable size into a larger screen and back.1
Overview
The enabling part is a flexible OLED touchscreen that bends around a precision hinge without cracking. Two broad styles have emerged: book-fold designs that open from phone size into a small tablet, and clamshell (flip) designs that fold a full-size phone down to a compact square. Inside, the device is a normal phone — an SoC, radios, and a battery split across the two halves — running an ordinary mobile OS adapted to switch layouts as the screen folds.
Where it fits
The foldable tries to collapse the phone-versus-tablet trade-off into one device: pocket portability when closed, more screen when open. The costs are mechanical complexity, a visible crease, added thickness, and a higher price — engineering challenges centered on durability of the hinge and the flexible panel over thousands of folds.
Sources
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Foldable smartphone — Wikipedia, on foldable phone designs and flexible displays. ↩