Specific gravity
Specific Gravity (SG) is a measure of the relative density of one material compared to another. For brewing, the reference material is water, so the specific gravity of wort or beer is simply the density of the liquid divided by the density of water. This results in a dimensionless number, which by brewing convention is usually listed to three decimal places. For example, pure water would have a specific gravity of 1.000, while an unfermented wort might have a specific gravity of 1.048. Finished beer would typically have a specific gravity in the 1.005-1.015 range. The specific gravity measure is somewhat ambiguous, since the density of water does vary with temperature, but by convention specific gravity in brewing is typically measured by a hydrometer calibrated to 60 F. The measured value should be adjusted based on the actual temperature of the measured fluid. Specific gravity is typically measured at several points during the brewing process - after sparging, before fermentation and after fermentation. The gravity measured before fermentation is called the Original Gravity and the measurement after fermentation is called the Final Gravity.
