A hidden single is a digit with only one place left in a house.
Each of the four windows holds 1 to 9 once, like a box. They are the houses you are least used to scanning, so singles hide there.
Worked examples
2 positions from real Windoku puzzles on this site, the familiar case first. Step through each one: the board lights what each step talks about, and the arrows show what rules what out.
One place left in a box
Look at box 7. The digit 1 must go somewhere in this box. Where can it go?
One place left in a window
The lit window is a house of nine cells. It must hold every digit once, like a box.
Where it fits
MediumHidden Single is a Medium technique: Medium Windoku puzzles are the first to need it.
The Classic Hidden Single lesson teaches it on a plain 9×9 board. The Windoku overview explains the rule and the levels.
Hidden Single in the other styles: Sudoku X, Jigsaw, Thermo, Arrow, Kropki.
Try it on a Medium Windoku puzzle. Next Step points it out when it is the move.