A naked single is the simplest thing a sudoku board can tell you: one cell, and every other digit is already ruled out by its row, column, or box, leaving exactly one possibility. “Naked” just means you’re looking at the cell’s own remaining candidates directly, not inferring anything from where else a digit could go.
What’s actually happening
Every empty cell has up to nine candidate digits — whatever hasn’t already appeared in its row, its column, or its 3x3 box. Most of the time a cell starts with several. A naked single is what’s left once enough of its row, column, and box are filled in that only one digit survives all three checks at once.
It’s the technique every solver uses without naming it. The reason it gets a name at all is that a difficulty rating needs one: a puzzle that can be finished using nothing but naked (and hidden) singles is, by definition, as easy as sudoku gets.
Worked example
This board is the exact state our logical solver was looking at the moment it found a naked single — mined from a real generated puzzle, not drawn by hand.
r7c6 has only one candidate left: 5.
Toggle “Show after” to see the digit go in. Nothing else on the board changes from this one placement — that’s what makes it the easiest kind of step: no elimination, no comparison between cells, just one cell whose candidates narrowed to one.
How to find one quickly
Don’t scan cell by cell. Scan by what’s missing: pick a row that already has six or seven digits filled in, and check its one or two empty cells against the digits still missing from that row. If a cell’s row, column, and box together rule out everything except one digit, you’ve got it. Boxes that are nearly full are the fastest place to look, since they constrain a cell from three directions.
Related techniques
Naked singles and hidden singles are the two techniques every Easy puzzle on this site is guaranteed to be solvable with — and nothing harder. Once a board stops giving you naked singles, the next thing worth checking is locked candidates, which works on candidates across a whole row or box rather than one cell at a time.
Practice spotting these on an Easy puzzle, or see the full rundown of how we grade difficulty on how difficulty is rated.