Naked Single
If you can eliminate all candidates for a cell except one, the cell must contain the remaining value.
Consider the sudoku below.
Looking at the cell r3c4, we can eliminate candidates 3, 6, and 9 because they are in row 3. We can also eliminate candidates 1, 2, and 8 because they are in column 4. We can also eliminate candidates 4 and 5 because they are in the same block as r3c4. This leaves 7 as the only candidate for the cell r3c4, so it must contain 7.
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