Foldable smartphones (also known as Foldable phones or simply Foldables) are a family of smartphone form factors utilizing hinged flexible displays.
In some cases, foldables are smartphones that can open up to become tablets, as we've seen with devices like the Galaxy Z Fold 2 and the Huawei Mate XS. In others, foldables are smartphones that can collapse down into a smaller form-factor, almost like flip phones. The Galaxy Z Flip and new Motorola RAZR are the best examples of this design.
Precursors to the concept have used multiple touchscreen panels on a hinge in a similar manner, but the term is currently synonymous with implementations that utilize a flexible display; concepts of such devices date back as early as Nokia's "Morph" concept (2008), and a concept presented by Samsung Electronics in 2013 (as part of a larger set of concepts utilizing flexible OLED displays), while the first commercially available folding smartphones with OLED displays began to emerge in November 2018.
The first generation of commercially-released foldable smartphones have been faced with concerns over their durability, as well as their high prices.
See more at "Form factor of mobile phones".