Turbidity

Here's an extreme example - perfectly clear water = low turbidity.  Muddy water = high turbidity.

 Turbidity is the measure of relative clarity of a liquid.  Turbidity makes water cloudy or opaque. You'll more often see higher turbidity levels on moving water (think, rivers) and less so on filtered water.

Particulates that can cause turbidity include:

• Algae
• Sand
• Silt
• Clay
• Precipitated iron
• Bacteria
• Fine organic matter
• Soluble colored organic compounds

Excessive turbidity, or cloudiness, in drinking water is aesthetically unappealing, and may also represent a health concern. Turbidity can provide food and shelter for pathogens. 

If not removed, the causes of high turbidity can promote regrowth of pathogens in the water, leading to waterborne disease outbreaks, which have caused significant cases of intestinal sickness throughout the United States and the world. 

Although turbidity is not a direct indicator of health risk, numerous studies show a strong relationship between removal of turbidity and removal of protozoa. The particles of turbidity provide "shelter" for microbes by reducing their exposure to attack by disinfectants. 

Microbial attachment to particulate material has been considered to aid in microbe survival. Fortunately, traditional water treatment processes have the ability to effectively remove turbidity when operated properly. 

If you fill a glass of water and it is clear, your turbidity is likely low.  If it seems murky, let it sit for a bit, it might just be air that needs to dissipate.  If the murkiness persists, you might have higher turbidity levels caused by contaminants in your water.

There are a few ways to treat for high turbidity in water.  For a home system, a Reverse Osmosis system is probably the easiest method.  They can run from around $100 up into many thousands of dollars, depending on capacity and other factors.  

Here's a link to a selection of RO systems at Amazon.com.  (I do not get an affiliate commission, this is just providing information for you.)

U.S. Geological Survey