Conductivity is typically mentioned in relation to electricity. Water has the ability to pass electrical currents, and conductivity is a measurement of how effective water is at passing those currents.
Conductivity is not static, which means conductivity can vary from water sample to water sample. It is measured by placing two electrodes in water and measuring how much electricity passes between them. The results are measured in micromhos per centimeter.
Electrical current flows through water by riding on the ions in that water. The water itself is not the medium.
Ions vary according to the other chemicals present in water in addition to the hydrogen and oxygen that make up water. Certain ions, such as nitrates, chloride and phosphates, are better carriers than sugar, alcohol and oil.
Water that has more active carriers is considered to be more conductive than water with fewer active, or inert, ions.
Heat speeds up the activity of ions, so the warmer the water, the more conductive it is. Testing is done at a standard 25 degrees Celsius to determine an objective reading.