A naked single occurs when a cell has only one possible candidate.
Also called "Sole Candidate" or "Forced Digit".
When all other digits 1-9 are eliminated from a cell, only one remains.
This happens when the cell's row, column, and box already contain 8 different digits.
The remaining digit must go in that cell.
Naked singles are easy to spot if you maintain candidate lists.
They're "naked" because the single candidate is obvious.
If a cell's candidates are reduced to just "5", then 5 must go in that cell.