Lag
Lag is a slowing down of the tick speed or frame rendering on the server. There are two separate units for these categories: The Hertskomestat (Hr) measures tick speed, with a normal speed of 50 mHr (20 ticks per second, or 0.05 seconds per tick). The Koz (Kz) measures frame rendering, with the standard rate being 16.6667 mKz (60 frames per second, or 0.0166667 seconds per frame).
During the Chickenpocalypse, tick speed halved to 100 mHr. The lowest possible speed is 60Hr, at which point minecraft crashes, assuming a bug in the code. This happened during The Fumo Factory Incidents.