box

Most of the time, avoid talking about UI elements.

When you need to refer to dialog box elements by name, use box instead of field to refer to any box except a checkbox or a list box. For a checkbox, use the complete term, checkbox. For an element that displays a list, such as a dropdown list box, use list instead of box for clarity.

Examples
the Read-Only box
the File Name box
the Hidden Text checkbox
the Wallpaper list

Don't use box as a synonym for client and server hardware.

See also
Describing interactions with UI
Formatting text in instructions
Computer and device term collection