What is defined as the process of one substance combining with another?

The process of one substance combining with another is best captured by the term "compound." A compound is formed when two or more different elements chemically bond together in a fixed proportion, resulting in a substance with unique properties that differ from those of the original elements. This definition highlights the chemical nature of the interaction where the substances undergo a transformation during the combining process, leading to the creation of a new entity with distinct characteristics.

While a mixture involves the combination of substances without a chemical reaction, allowing them to retain their individual properties, and a solution describes a specific type of mixture where one substance is dissolved within another, these terms do not reflect the transformation and permanence implied in the creation of a compound. Thus, the process referred to in the question is more accurately defined as the formation of a compound, making it the appropriate choice.