I’ve seen this a few times.
See the glossary of terms found here to answer your question. 
It can also stand for “Mixed Member Proportional”, an electoral system used in New Zealand, Germany, Scotland, Boliva, Lesotho, and Mexico, for what it’s worth…
Since this is a question about this message board let’s move it to…hmmmm?
samclem GQ moderator
Hey you!
This week we’re over here.
Feel free to join in. You don’t have to read the whole thing to join in. Hijacking the thread is pretty much the point. Oh, and chocolate. Chocolate is very important to us.
GT