Here's why:

* Saturated: A fatty acid is saturated if all the carbon atoms in its chain are bonded to the maximum number of hydrogen atoms possible. This means there are no double bonds.

* Unsaturated: A fatty acid is unsaturated if it contains at least one double bond between carbon atoms in the chain.

* Monounsaturated: This specifically refers to a fatty acid with only one double bond.

* Polyunsaturated: This refers to a fatty acid with two or more double bonds.

So, a fatty acid with one double bond is monounsaturated.