What is defined as a naturally occurring, inorganic solid substance with a definite composition?

The term that accurately describes a naturally occurring, inorganic solid substance with a definite composition is "Mineral." Minerals are typically formed through geological processes and possess specific physical properties such as hardness, luster, and crystal structure. Each mineral has a unique chemical formula that dictates its composition, setting it apart from other substances.

In contrast, magma refers to molten rock located beneath the Earth's surface, and lava is magma that has reached the Earth's surface. Both of these are primarily in a liquid state and do not fit the criteria of being solid, inorganic substances. An ion, on the other hand, is an atom or molecule that has gained or lost one or more electrons, giving it a positive or negative charge, and does not represent a solid substance with a definite composition. Thus, "Mineral" is the only choice that correctly aligns with the definition provided in the question.