Adaptive can mean different things.

In an adaptive limiter the adaptive part can refer to frequency or timing. This is usually done in a look ahead manner which allows the limiter to anticipate the changes it needs to make to the signal to keep it within the parameters you have set for it.

There were a couple of plug ins that did this along with a couple of pieces of hardware that also did this. It is a way of getting the maximum level from your music without going into overload or distortion. I have never personally used one but the concept is a good one. Probably hard to implement properly.

Hope this helps!

Like what Thomas said. Basically it looks at the signal and changes it's parameters based on the signal to hopefully sound better.

I dont understand how a limiter can look ahead in real time.
It seems that the math involved in anticipating the changes in something as complex as an audio waveform would be above and beyond what a pc is capable of.
Or am I looking at this from the wrong angle?

See here

http://www.omniaaudio.com/tech/lookahead.htm

Thank you guys!