What is a combination of substances in which each substance retains its own properties?
Mixtures are, as you've defined, combinations of substances in which each substance retains its own properties. A salad, for example, contains several individual food components, but each retains its own chemical properties.
This is different from a compound, which has different chemical properties from its original elements. One of the best examples of this is table salt, or NaCl. Sodium and Chlorine are both highly reactive (and dangerous) elements, but when they come together, they form a compound that is not only docile, but that we actually consume everyday.
Hope that helped :)