Selectable is an option for boxes, cells and notebooks which specifies whether their contents can be selected interactively using the front end.
Even with the setting Selectable->False, an object can be selected as a whole.
With Selectable->False set at the notebook level, no cells in the notebook can be selected.
See Section 2.9.10 and Section 2.11.9.
See also: Editable, WindowClickSelect, StructuredSelection, ShowSelection.
New in Version 3.