A homogeneous substance is a substance that appears to be the same throughout at high magnification and has no visible phase boundaries inside it. A homogeneous substance can be a mixture, a compound or an element.
An example of a homogeneous mixture is salt water in which the salt has completely dissolved. In this case, no matter how small an amount of the salt water is taken, the basic composition and appearance of the salt water remain uniform. In contrast, oil in water is a heterogeneous, or nonhomogeneous, mixture. In this case, it is possible to see clear phase boundaries between droplets of oil and the surrounding water.