In hockey, the period from which a team becomes short-handed due to one or more penalties until they are at full or even strength with the opposing team. Also refers to the defensive activity during such a period.
When a team has a one man or more advantage over another team in hockey due to a penalty, this condition is referred to as a "power play." Conversely, the team that is minus one or more players (also referred to as "short-handed") is often referred to as being in "penalty kill" mode. What this means, essentially, is that the team that is short-handed should be primarily focusing on defense until the penalty or penalties assessed to them have expired and they are restored to full strength. The disadvantaged team is basically "killing" the penalty by denying the opposing team the opportunity to score when they have the upper hand.
"Penalty kill" is often abbreviated to PK in both written and colloquial form when referring to hockey. Teams are often rated on their ability to successfully defend during penalty kill periods. Because it is often difficult to avoid penalties entirely, penalty kills are a necessary component of the game for teams to master if they want to stay competitive.