Hidden Single
A Hidden Single is a digit that has only one possible cell left in a house — even though that cell might hold other candidates too. The digit is "hidden" because it's not alone in its cell; it's alone in the house. This is the first technique that asks you to look at candidates instead of just counting filled cells, and it shows up constantly throughout any solve.
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
Go through the houses one at a time — row, column, box — and for each digit, count how many cells could still take it. If only one cell can, that's a Hidden Single. Pencil marks speed this up enormously: if a digit's mark shows up in only one cell of a house, that's your placement. Boxes are usually the fastest to scan because the nine cells sit in a compact 3×3 block. After a while you start seeing them without even checking notes — your eye just catches that a digit is missing from a house and there's only one hole left for it.
The logic
Each digit has to appear exactly once in every row, column, and box. If a digit has only one possible cell left in a house, it has to go there — there's nowhere else. That's the whole argument. It's the natural step up from Last Free Cell and Last Remaining Number, because it works even when a house still has plenty of empty cells. It also trains the habit of thinking in candidates rather than just counting filled cells, which every harder technique builds on.