Final answer:
The correct term for a substance 'composed of two or more different atoms' is a compound, which consists of atoms of different elements chemically bonded together.
Step-by-step explanation:
The correct word that completes the sentence "composed of two or more different atoms" is compound. A compound is a substance that is made up of more than one type of atom bonded together. This is distinctly different from an element, which is made up of only one type of atom. For example, water (H₂O) is a compound consisting of hydrogen and oxygen atoms combined in a fixed ratio, whereas oxygen gas (O₂), as an element, is composed of oxygen atoms only.