Homogeneous Mixture
A homogeneous mixture has the same uniform composition throughout, so you cannot see its separate parts; solutions like salt water are examples.
Definition
A homogeneous mixture looks the same all the way through because its parts are mixed so evenly that you cannot see them separately. It has a uniform composition and appearance, with the components mixed at the level of individual particles; solutions are the most common examples and can be solid, liquid, or gas, and the components remain unbonded and physically separable by evaporation or distillation. Strictly, it is a single-phase system in which the components are dispersed at the molecular or ionic scale, so intensive properties such as density and refractive index are identical at every point. Alloys, aqueous solutions, and gas mixtures all qualify, and the distinction from heterogeneous mixtures is one of scale: a mixture may be homogeneous macroscopically but heterogeneous at nanometer resolution, as with colloids.
Example
Sugar stirred into tea disappears, and every sip tastes equally sweet. Brass is a solid homogeneous mixture of copper and zinc, air is a gaseous one, and vinegar is a liquid one, about $5\%$ acetic acid in water, with every drop at the same concentration. A $1.00\text{ M}$ sodium chloride solution contains $58.44\text{ g}$ of $NaCl$ dissolved in enough water to make $1.00\text{ L}$; the $Na^+$ and $Cl^-$ ions are surrounded by water molecules and distributed evenly, so any $10\text{ mL}$ sample contains $0.0100\text{ mol}$ of salt.
Key Insight
"Homo" means same, so if every spoonful is the same as every other spoonful, the mixture is homogeneous. Uniform does not mean pure: salt water is uniform, but it is still two substances, and boiling the water off leaves the salt behind. Some alloys are homogeneous solid solutions, like brass, while others such as cast iron contain distinct phases visible under a microscope, so whether a mixture counts as homogeneous can depend on the magnification you use.