available area; usable area
Computing term referring to a region of memory, disk, or screen that is currently free or accessible for use.
This program automatically detects the available area.
The available memory area is insufficient.
空き領域 is a more general term for 'free space' or 'empty area', while 利用可能領域 emphasizes that the area is available for use, often in a technical or computing context.
Compound of 利用可能 (available, usable) and 領域 (area, region). A straightforward technical term.