Geo-stigma and Star-Scar Syndrome
This was released in the Vince Summary on the adventchildren.net webpage....
Vincent: 'Geostigma' is when your body has overworked itself purging wicked substances from within itself.
Cloud: Wicked substances...
Vincent: There's something that resembles the lifestream flowing within our bodies, and that's what's fighting the evil substances that have intruded within. The Calamity from the Skies....Jenova.
So Geo-stigma is your bodys reaction to purging the Jenova cells within your body.
Kadaj also says it:
Kadaj: You see, someone hid 'Mother' away from us. The 'stigma is an illness that awakens within you the action of compensating where the body itself has reached its limit. I'm sure...you know very well yourself exactly what this 'stigma is, don't you?
Stigma is defined as: A small mark; a scar or birthmark.
And Geo is defined as: Earth
Something else I found - a translation of the TGS video commentary...
Due to the valiant efforts used to save the world two years before, the world was thought to be peaceful once and for all. However, a new illness known as the Star Scar Syndrome started to spread, and the people began to suffer and die off.
And Cloud—who fought alone because the world refused others to live—lived a lonely life.
Voice of a Small Girl: "That day, the power was used. Therefore, the star now lives. The humans have become the star, their souls among the star. And I had thought it would all be over..." (Translation Note: 'Star' may be referring to The Planet.)
Even though its not mentioned we know what the scar is... and if you don't... you dont deserve to see the movie :p