Selectable

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 The Mathematica Book: Section 2.8.10 and Section 2.10.9.

See also: Editable, WindowClickSelect, StructuredSelection, ShowSelection.