"Portable screen" and "mobile display" are terms that basically have the same meaning, but there are subtle differences.
"Portable screen" is a general term that simply refers to a movable screen or display. It is a term that can be applied to any type of device and can be used in any situation, such as a laptop screen or a mobile device screen.
On the other hand, "mobile display" is a term that refers to displays that are mainly related to mobile devices (smartphones, tablets, handheld game consoles, etc.). In other words, it often refers to displays that are mainly built into portable electronic devices.
In short, "portable screen" is a broad concept that focuses on the fact that the display itself is movable, while "mobile display" is a specific term that refers specifically to displays related to mobile devices.
