Hidden Pair
A Hidden Pair occurs when two digits are confined to a total of two cells in one row, column, or box. The digits need not both be written in both cells before the deduction. Other candidates can be removed from those two cells.
What is a Hidden Pair?
For two selected digits, take the union of all their remaining positions in one sector. If that union contains exactly two cells, those cells must hold the two digits between them, so no other candidates can remain in them.
How to spot it
Track where each digit can go in one sector and find two digits whose combined positions occupy exactly two cells. An identical pair of candidate marks is common, but the position-union definition is the general rule.
The logic
Both selected digits must occur in the sector, and only the two selected cells can hold them. The cells are reserved for those digits, so every other candidate can be removed from the two cells.