Salinity is the measure of how much salt a sample of water contains. Salt is measured in parts per thousand (ppt). Typically saltwater has a measurement of 35ppt and a Specific Gravity of 1.025.
Specific Gravity is a denisty measurement of a substance in relation to the density of water. In the aquarium, the Specific Gravity is used to determine the salinity of the water. Pure distilled water has a specific gravity of 1.000, where as natural seawater typically registers at 1.025. However, saltwater fish are usually housed in water registering a specific gravity of 1.020 - 1.023. These measureents are determined using a Hydrometer.
The abbreviated term for Small Polyped Stoney Corals. These are calcium-based, either branchy or plated hard corals, with small polyps covering their skeletal form.