Hidden Single
A digit you place because it has nowhere else to go in a row, column, or box — even though that single cell might still have several candidates of its own.
5
5
5
5
Where can the 5 go?
Look at the top-left box. We're hunting for the digit 5:
- Row 0 already has a 5 (column 5) → kills three cells in the box.
- Row 2 already has a 5 (column 7) → kills three more.
- Column 0 already has a 5 (row 4) → kills (1, 0).
- Column 2 already has a 5 (row 5) → kills (1, 2).
Eight of the box's nine cells are blocked. Only the cell highlighted in amber can hold the 5.
Naked vs Hidden
Naked Single: the cell only has one candidate.
Hidden Single: a digit only has one cell — the cell may still admit other candidates, but those will be ruled out later.