Hidden Single

Sometimes a cell has several candidate digits, so it doesn't look special on its own. But if you scan a whole row, column, or box for where a particular digit could go, and it turns out only one cell in that unit can hold it, that digit must go there.

It's 'hidden' because the single is hiding among other candidates in the cell — you find it by looking at the digit across the unit, not by looking at the cell in isolation.

Picture a row where 7 hasn't been placed yet, and 7 is still a candidate in three of its cells. If two of those three cells sit in a box that already has a 7 placed elsewhere, that leaves exactly one cell in the row where 7 can go — even though that cell also lists other candidates alongside it.