Is salt a heterogeneous mixture, a compound or a homogenous mixture?

I believe, by salt you are referring to common salt or the salt we commonly use at home. This salt is chemically known as sodium chloride and has a chemical formula of NaCl. It is made up of atoms of two different elements, sodium and chlorine, a fixed 1:1 ratio. That is, each atom of sodium combines with an atom of chlorine in sodium chloride or common salt. Also, the properties if each individual atom is different than the properties of the salt molecules. Hence, common salt is a compound. Compounds are made up of elements, combined in a fixed ratio and has different properties than constituent atoms. Common salt exhibits both the characteristics.

In comparison, mixtures are made up of elements, where each element still retains its properties and it is easier to segregate the constituents.

Hope this helps.