Hidden Single (box)

Instead of asking what a cell can hold, ask where a digit can go. Take a box and a digit not yet placed in it, and check the nine cells. If eight of them are blocked — by that digit already appearing in their row or column, or by being filled — the ninth must be it.

This is called hidden because the cell may still have several candidates written in it. The digit is not the only thing that cell could take; it is the only cell that digit can take. Solvers spot box singles fastest, because a box is small enough to take in at a glance, which is why they are graded as slightly easier than the same deduction along a line.