Example of a Double
This is a screenshot
Double: Two Squares in the same Row, Column, or Box that contain the same two Possibles - and no others.

In this example, the Double is the two Squares containing the Possibles 7 and 8 (coloured green). These two Squares MUST contain 7 or 8. Therefore 7 and 8 cannot be in any other Square in the same Row or Box. The 8 outlined in orange is an impossible solution and can be removed.