Refractometer
Optical instrument for measuring the salinity of sea water. More precise than the hydrometer, it is considered the minimum standard for a reef aquarium.
Detailed explanation
The refractometer measures the refractive index of light passing through a drop of water. From the refraction it calculates the concentration of dissolved salts and converts it into salinity (ppt) or density (SG).
It works simply: you put a drop of water on the prism, close the lid, look into the eyepiece and read the value on the scale. The dividing line between the light and dark areas indicates salinity.
Calibration must be done regularly with pure osmosis water (which should read zero) or with a reference solution at 35 ppt. Seawater compensated (ATC) refractometers are preferable because they automatically correct for temperature.
The refractometer costs between 20 and 50 euros and is infinitely more precise than the float hydrometer (the plastic one you buy in aquarium shops). The hydrometer has huge margins of error and air bubbles on the float distort the reading.
For the reef, precision in salinity is critical. A difference of 2 ppt (for example 33 vs 35) can significantly stress corals and invertebrates. Water changes with salinity different from that of the aquarium cause osmotic shocks.
Digital refractometers (like Hanna HI96822) are even more precise but cost much more. For most aquarists a good optical refractometer is more than sufficient.
In brackish waters the refractometer is equally useful for checking that the salinity remains within the desired range.