A hidden single occurs on a sudoku board when there is only one cell left in a row, column, or box where a specific number can occur. Sometimes they are very easy to find, and sometimes they are not as obvious.
![orange cell is a hidden single](https://sudokubeginner.com/wp-content/uploads/2016/05/Sudoku_Hint_Generator_–_Sudoku-1-300x298.jpg)
When using pencil marks, you can visually scan through a row, column, or box. When you see that a number only occurs once, that means you have a hidden single.
![hidden single with pencil marks](https://sudokubeginner.com/wp-content/uploads/2016/05/Sudoku_Hint_Generator_–_Sudoku-2-300x300.jpg)
![hidden single with pencil marks highlighted](https://sudokubeginner.com/wp-content/uploads/2016/05/Sudoku_Hint_Generator_–_Sudoku-3-300x300.jpg)