Final answer:
Seawater with a salinity lower than 35 is referred to as brackish, which occurs when seawater mixes with fresh water from rivers or precipitation.
Step-by-step explanation:
Seawater typically has a salinity of about 35 parts per thousand, which is sometimes expressed as 35 practical salinity units (PSU). When bodies of seawater have a salinity lower than 35, they are often referred to as brackish. Brackish water is usually the result of the mixing of seawater with fresh water from sources like rivers or precipitation. This variation in salinity can have significant impacts on the marine environment, affecting the types of organisms that can thrive in different water bodies.