Definitions of a Chemical Compound:
A compound is the result of two or more elements becoming chemically combined together. The elements react chemically, causing bonds to be formed between the atoms involved ... resulting in molecules of the compound. The elements react in specific amounts, resulting in a compound that has a specific chemical composition. Compounds do not necessarily resemble the elements of which they are composed and cannot be converted back to those elements by physical methods. It can even be very difficult and require a lot of energy to change compounds into their elements by chemical means (reactions).
A compound is a pure substance that consists of two or more elements chemically combined in a fixed proportion, that can be further subdivided into simpler substances by chemical (not physical) means only. ... So, if a quantity of a material consists of atoms of two or more elements joined together, always in the same ratio, then the matter forming that material is a compound.