Naked Pair

If two cells in the same row, column, or box both have exactly the same two candidate digits and nothing else, then those two digits must occupy those two cells — we just don't know which one goes where yet.

Either way, no other cell in that unit can hold either digit, so both candidates can be eliminated from every other cell in the unit.

A naked pair only counts if both cells have exactly those two candidates and nothing else — a cell listing {3, 7, 9} doesn't form a pair with one listing {3, 7}, even though they overlap. Mistaking 'shares some candidates' for 'shares exactly two candidates' is the most common way to misapply this technique.