A homogeneous mixture is a (solid, liquid, or gaseous) mixture that has the same ratios of its components in any given sample.
On the other hand, a heterogeneous mixture has components whose proportions are different throughout the sample.
"Homogeneous" and "heterogeneous" are not absolute terms but depend on context and the size of the sample.
In chemistry, a homogeneous suspension of material means that when dividing the volume in half, the same amount of material is suspended in both halves of the substance; however, it might be possible to see the particles under a microscope.
