What is a compound?

I- A compound is a substance that can be decomposed into simpler forms of matter by conventional chemical changes.
II- Compounds are composed of two or more elements.
III- Compounds have a constant, defined, and invariable composition.
IV- Compounds can be observed, identified, and manipulated in macroscopic quantities.
V- Examples of compounds are water (H2O), common salt (NaCl), and calcium carbonate (CaCO3).