* Heterogeneous mixture: A mixture where the components are not evenly distributed and can be visually distinguished.

Let's examine each option:

* Water: Water is a pure substance, not a mixture.

* Saltwater: Salt dissolves completely in water, creating a homogeneous mixture (evenly distributed).

* Dirt: Dirt contains various components like sand, rocks, organic matter, etc. These components are not evenly distributed, making it a heterogeneous mixture.

* Brass: Brass is an alloy, a homogeneous mixture of metals (typically copper and zinc).

Therefore, dirt is the only example of a heterogeneous mixture from the list.