hysteresis?

What exactly does it mean? I cant seem to get a good definition from context clues.:banghead:

its basically the time/units needed to activate or deactivate something...the actual definition of it is

The lagging of an effect behind its cause,

but i doubt thats what your looking for, but as an example lets say your talking about the rev limiter. Lets say the hysteresis is 300 rpms well If the rev limiter is set to 6300, once you hit that even with your foot to the floor the engine will not attempt to rev again until 6000 rpms. 6300-300(lag time)=6000 hope that explains it for you.

Hysteresis used to go by the more descriptive
"dead band". Think of it like a sloppy linkage.
If you are pulling and want to push you have to
first take up the slack. The slack is your hysteresis.
It's useful in controls because you don't want to be
switching like mad right when you are where you
want to be. So hysteresis would like to be about
the width of your acceptable tolerance or maybe
a little less, so you just "stay between the lines"
and don't worry about twitching the wheel every inch
of wander.

Thanks guys, that pretty much explains it for me.

The analogy of the sloppy linkage is a great one. Here is one example of why hysteresis is used:

Lets say you switch from Fuel trim cell 12 to 13 at 2000 RPM. If your cruising right at 2000 RPM, your actual crank speed as measured by the crank sensor will probably vary from 1975 to 2025 RPM. So there may be be a 50 RPM hysteresis in either direction to prevent the fuel trim cells from toggling back and forth, which could cause a drivability issue. So if 2000 RPM is the dividing line with a 50 RPM "buffer", then it won't go to cell 13 until 2050 RPM, nor will it go back to cell 12 until 1950 RPM.

So, i think i had this problem when screwing around with my PE. It would bump in and out of PE on the interstate, so i had to go really really fast, or really really slow to get the revs to quit going from 2K to 3K, and vice versa....Is that an instance of maybe 2 tight of a hysteresis restriction...or however you'd word that?

excellent explanation. Ive wondered what this term meant in real world terms for a while now, thank you.