A rather difficult skateboarding trick. It is usually performed on vert, without much rotation. The madonna was invented by some guy named Tony Hawk, when he was in the business of inventing some 90 or so tricks. He wanted to give it a name that was new and trendy, so he looked at what was up and coming. Guess who was getting her start at the time? (Hint, look at the writeup above this one, genius)
What a madonna consists of is, after ollieing, you reach down with your trailing hand, and grab the tail of the board, like a tailgrab. Now, take your trailing leg off of the board, and lift the board up with your hand, and your leading leg at the same time. The board should now be held by your hand and your leading leg, and would be rotated about 90 degrees to how it was previously. Thrust your free leg as far away from your shoulders (down) as possible, and you just tweaked it.
If you want to see a demonstration, you can either go to a vert ramp and ask somebody to do one, or else just play some Tony Hawk Pro Skater.