Salinity is a measure of the total amount of dissolved salts in seawater. It is measured in parts per thousand (ppt or 0/00). The average salinity of the ocean is around 34 to 37 ppt.
Marine aquariums are also measured in specific gravity. Specific gravity may be defined as the ratio of the density of the liquid to the density of water. Since density of liquid varies with temperature, so does specific gravity. It has been determined that liquids with a specific gravity less than 1 are lighter than water; those greater than 1 are heavier than water. The specific gravity of seawater at 35 ppt is 1.026.
Because of evaporation, the salinity in your tank will change continuously. Salt does not evaporate, only water. This causes the salinity to go up. To avoid salinity fluctuation, be sure to top off your tank with fresh water as often as possible.