Zed Time is a gameplay mechanic in the Killing Floor franchise that slows down time temporarily. This is triggered randomly after killing Specimens. The slowed down time allows you to better line up your attacks, or just admire the gore flying around you. It also has an annoying tendency to occur when you're reloading and can't take advantage of it.
In the first game the Commando and the Berserker both gain the ability to extend Zed Time after they have leveled up some. In Killing Floor 2, all Perks gain Zed Time-related abilities at level 25.
There is also a Weekly Outbreak that puts the game into Zed Time for the entire match, but disables level 25 skills.
The idea for zed time came about when Tripwire Interactive president John Gibson woke up one morning and the first thing that popped into his head after he opened his eyes was "slow motion zombie killing."