Pointing Pair
Intermediate Removes candidates Used in: Sudoku, Killer Sudoku
Inside one box, all candidates for a digit lie on a single row (or column). The digit must therefore end up on that line within the box, which means it cannot appear anywhere else along the same line outside the box — remove it there.
How to spot it
Inside a box, check whether all candidates for a digit fall on one row or one column.
What you can conclude
Delete that digit from the rest of that line outside the box.
Worked example
In a box, 3 can only sit in its top row, so 3 leaves the rest of that row in the neighbouring boxes.