A 4-minute penalty given to a player for certain infractions of the rules that don't warrant a major or match penalty. This only includes butt-ending, head-butting, high-sticking and spearing. High sticking is the most common double-minor penalty.
Unlike a major penalty, the time in a double-minor penalty can be reduced due to the scoring of a goal by the team on the powerplay.
For example, if the team on the powerplay scores in the first half of the penalty, the time goes down to 2 minutes and the player remains in the penalty box. If the team scores in the final 2 minutes, the player can return and the penalty is terminated.
Also, keep in mind, a player could receive two separate penalties during the same play resulting in 4 minutes on the penalty clock. Although this resembles a double-minor penalty, it's not a true double minor. However, it does behave the exact same as a double-minor penalty when the team at extra strength scores.