Hidden Single (Unique Candidate)
If a cell is the only one in a row, column or block that can take a particular value, then it must have that value. All rows, columns and blocks, must contain each of the digits 1 to 9, so if there is only one cell that can hold a particular value, then it must hold that value.
For example, in the partial Sudoku puzzle below, the marked cell is the only one in block five that can hold a 2, and so it must hold a 2.
So why is this technique called hidden single? Simply because if you use a computer assistant (such as SadMan Sudoku) that gives you the full and complete candidate listing for all cells, these cells are the only ones to have a certain digit each, but they're hidden amongst the other candidates for the cell. Contrast this to naked single.
Here are some Sudoku puzzles that can be solved using only naked and hidden singles: (What are .sdk files?)
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Houle Bækdal, Denmark