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.
Importance at Expert Level
Hidden Pairs are frequently 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.