Two or more atoms joined together is defined as:

The definition of two or more atoms joined together fits the concept of a molecule. A molecule is formed when two or more atoms bond together, which can consist of atoms of the same element or different elements. For instance, oxygen gas (O₂) is a molecule composed of two oxygen atoms, while water (H₂O) is a molecule made up of two hydrogen atoms and one oxygen atom.

In contrast, a compound specifically refers to a substance formed from two or more different elements that are chemically bonded, such as water. While all compounds are molecules, not all molecules are compounds if they consist of only one element. An element represents a pure substance made up of only one type of atom, like hydrogen or carbon. An ion refers to an atom or molecule that has gained or lost one or more electrons, resulting in a net electrical charge. Thus, the best definition for the phrase "two or more atoms joined together" is indeed a molecule.