Mirrored from Sudopedia, the Free Sudoku Reference Guide
The Hidden Single is a lot less sinister than its name suggests.
In fact, most of the Sudokus published in newspapers and magazines can be solved by hidden singles only. Together with the Naked Single it belongs to the singles, the most elementary solving techniques.
An alternative term is Pinned Digit.
It may sound like a daunting task, checking 27 houses for each of the 9 digits, thus performing 243 checks, but most players do not check each of these combinations individually. Instead, they use techniques like Scanning and Cross-Hatching to locate the hidden singles.
Some Sudoku Helpers, such as SudoCue and Simple Sudoku, have the ability to highlight the cells for a chosen digit. Such highlighting makes Hidden Singles particularly obvious. The following is the same example grid with all cells having 4 as a candidate highlighted.