CellOpen

CellOpen is an option for Cell which specifies whether the contents of a cell should be explicitly displayed.

With CellOpen->False, a small cell bracket is still shown to indicate the presence of a cell.

Cells which are not open can still be evaluated automatically if you set InitializationCell->True.

See The Mathematica Book: Section 2.10.9.

See also: Visible, CellGroupData, ConversionRules.