Specific gravity....gotta love this one. Pure water is the reference: 1.000. Distilled for us common folks. Weight to volume, since I don't live any where else, that is here on Earth. Solution: a mixture. In this case, water and sulfuric acid. The concentration of acid in solution can give an indirect measurement of the charge in a battery. The measurement of "specific gravity" tells you what the concentration of sulfuric acid is at the time you measure it. That can tell you what the charge is of the battery. If you would like to know what those numbers really mean try this.
http://www.engineersedge.com/battery/specific_gravity_battery.htm

If you are like me and I mean no disrespect for that. Follow the book when measuring a battery. Enjoy

Steve H