The speed required for the Time Machine, in the Back to the Future series, to travel through time. The time shift would only occur slightly after hitting the 88 mile an hour mark; time travel under this speed was impossible under normal conditions.
The lightning strike at the end of Back to the Future Part Two forces the Time Machine to travel through time, even though it's practically at a standstill. However, the flux capacitor was overloaded from this lightning strike, essentially allowing it to bypass the required 88 mile an hour requirement. Under normal use (non-flux capacitor-overloading conditions) the Time Machine needs to obtain the 88 mile an hour speed to travel through time.
Why 88? Well, my only idea is that on a digital read-out, 88 is displayed when all the LEDs are lit up. When you reset digital items they sometimes display flashing 88's.
I'm also not sure if this is a minimum speed, or if it's the only speed at which the time machine can travel through time. Can it accelerate past 88 as it travels through time, or after it has traveled through time? No idea.