ShowCellLabel

ShowCellLabel is an option for Cell which specifies whether to display the label for a cell.

ShowCellLabel is more often set for styles of cells than for individual cells.

With the setting CellLabelAutoDelete->True, the label for a cell is automatically deleted if the cell is modified.

See The Mathematica Book: Section 2.10.9.

See also: CellLabel, ShowCellTags.