A hidden single is a digit with only one place left in a house.
In Kropki a digit can be eliminated from a cell by its neighbours rather than by a house: a dot whose partner the neighbour cannot hold, or a missing dot beside a digit one apart. So when a house seems to have two places left, look at the dots too.
Worked examples
2 positions from real Kropki 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 1. The digit 9 must go somewhere in this box. Where can it go?
A dot blocks the other places
Some cells of this house cannot take the digit because of the dots beside them, not because of their row, column or box.
Where it fits
MediumHidden Single is a Medium technique: Medium Kropki puzzles are the first to need it.
The Classic Hidden Single lesson teaches it on a plain 9×9 board. The Kropki overview explains the rule and the levels.
Hidden Single in the other styles: Sudoku X, Jigsaw, Thermo, Windoku, Arrow.
Try it on a Medium Kropki puzzle. Next Step points it out when it is the move.