Hidden Single
A Hidden Single is a digit with only one possible cell in a row, column, or box. The cell may have other candidates, though a candidate can be both hidden and naked. Pencilmarks help reveal the position but are not a logical requirement.
What is Hidden Single?
Every house has to contain each digit 1–9 exactly once. Sometimes a particular digit can only go in one cell of a house — every other cell is either filled already or ruled out by the digit appearing elsewhere in the same row, column, or box. That cell is the digit's hidden single. It's "hidden" because the cell itself may still have other candidates written in it; the digit isn't alone in the cell, it's alone in the house. Once you spot it, you place the digit and cross it off everywhere else that sees it.
How to spot it
Inspect one row, column, or box and track the remaining positions for each digit. If a digit has only one possible position, place it there. You may find this from written candidates or directly from the placed digits and earlier eliminations.
The logic
Each digit occurs exactly once in every row, column, and box. If only one cell in a sector can still contain a digit, that candidate is true. This is the same positional idea as Last Remaining Cell; when it is also the cell’s only candidate, it is both hidden and naked.