Hidden Pair
When two digits appear as candidates in only the same two cells within a unit, all other candidates can be removed from those two cells.
A Hidden Pair occurs when two specific digits exist as candidates in only the same two cells within a row, column, or block. Since those two digits must occupy those two cells, all other candidates in those cells can be eliminated.
Difference from Naked Pair
A Naked Pair is when "two cells have only the same two candidates," while a Hidden Pair is when "two digits exist as candidates in only the same two cells within a unit." The Hidden Pair cells may have other candidates as well, but since the pair of digits is locked in, the other candidates can be removed.
Tips for Spotting Them
Pick one unit and write down the digits whose candidate positions are limited to just two cells. If two of those digits share the same two cells, a Hidden Pair is formed. For example, if digits 3 and 7 can only go in R2C2 and R2C8 within a row, those two cells must hold 3 and 7, so every other candidate in them can be erased. Digits whose candidates are spread over three or more cells do not qualify, so check first that the positions are limited to two cells.
Importance at Expert Level
Hidden Pairs are needed at Expert difficulty. Finding them requires accurate pencil mark management and awareness of which cells contain each digit as a candidate within each unit.