"Sole Candidate" Sudoku solving technique explained:

sudoku showing the sole candidate technique

A Sole Candidate means that in a specific cell only one number remains possible.

This applies after eliminating all the other candidates by using different solving techniques. In other words, a sole candidate is the last remaining option and there cannot be any other option, therefore it’s the “Sole Candidate”.

Take a look at the example:

look at the block marked in pink, the cell marked with a blue square has only option - 6

sudoku showing sole candidate solution

It’s also known to some by the names "Naked Single" or "Forced Digit".