The Window of Opportunity is a mythological window where a newly empowered witch has 48 hours to decide whether to use their powers for good or evil. The witch can't be forced which side to choose, but both sides can entice him/her. According to the Oracle, it is "the great flaw in the Grand Design".
The Source tried to lure Paige, who was new to her powers, to the side of evil, instead of killing her. He almost succeeded, but Prue, Piper, and Leo managed to thwart his plan at the 47th hour.
Season 4 Fourty-Eight Hours