Hidden Single
A state where a specific digit can only be placed in one cell within a row, column, or block.
A Hidden Single occurs when a specific digit has only one possible placement within a unit (row, column, or block). The cell itself may have multiple candidates, but the logic "this digit can only go here" confirms the placement.
Difference from Naked Single
Naked Single narrows candidates from the "cell's perspective," while Hidden Single narrows placement from the "digit's perspective." Because it requires that shift in viewpoint, it is often described as harder to spot until you are used to it. How quickly you find one depends on your familiarity and on the state of the board, so it will not always come as fast as a Naked Single. The two are not exclusive; you can alternate between them on the same board.
Block Scanning Method
Focus on a specific digit and examine blocks where it has not yet been placed. Use row and column constraints to narrow down possible cells. If only one cell remains, a Hidden Single is confirmed. Scanning all blocks for each digit from 1-9 is the systematic approach. Digits that are already placed many times on the board give you more material for exclusion, so they tend to offer clearer clues for narrowing down.
It Also Holds for Rows and Columns
Hidden Single is not a property of blocks alone; the same logic holds for rows and columns. If only one empty cell in a row can take the digit, that cell is confirmed. When block scanning stalls, shifting your viewpoint to rows and columns sometimes reveals a placement.
A Common Misconception
The confirmed cell does not need to have its candidates narrowed down to one. Even if other candidates remain in that cell, you can confirm it as long as the digit has only one possible placement across the whole unit. Some assume the technique is unusable without writing pencil marks, but you can verify it just by looking at the board.