A hidden single is a digit with only one place left in a house.
In Thermo a digit can be eliminated from a cell by its thermometer rather than by a house, so look at both: the other places in a house may be ruled out by a digit nearby or by a thermometer that cannot take it there.
Worked examples
2 positions from real Thermo 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 3. The digit 7 must go somewhere in this box. Where can it go?
A thermometer blocks the other places
Some cells of this house are on thermometers. A digit can be eliminated there by its thermometer, not only by its row, column or box.
Where it fits
MediumHidden Single is a Medium technique: Medium Thermo puzzles are the first to need it.
The Classic Hidden Single lesson teaches it on a plain 9×9 board. The Thermo overview explains the rule and the levels.
Hidden Single in the other styles: Sudoku X, Jigsaw, Windoku, Arrow, Kropki.
Try it on a Medium Thermo puzzle. Next Step points it out when it is the move.