A hidden single is a digit with only one place left in a house.
In Arrow a digit can be eliminated from a cell by its arrow rather than by a house: a circle cannot be smaller than its arrow adds up to, and a cell on an arrow cannot push the sum past its circle. So when a house seems to have two places left, look at the arrows too.
Worked examples
2 positions from real Arrow puzzles on this site, the familiar case first. Step through each one: the board lights what each step talks about, and the red lines show what rules what out.
One place left in a box
Look at box 5. The digit 7 must go somewhere in this box. Where can it go?
An arrow blocks the other places
Some cells of this house are on arrows. A digit can be eliminated there by its arrow, not only by its row, column or box.
Where it fits
MediumHidden Single is a Medium technique: Medium Arrow puzzles are the first to need it.
The Classic Hidden Single lesson teaches it on a plain 9×9 board. The Arrow overview explains the rule and the levels.
Hidden Single in the other styles: Sudoku X, Jigsaw, Thermo, Windoku, Kropki.
Try it on a Medium Arrow puzzle. Next Step points it out when it is the move.