Which term refers to an increase by natural growth or addition?

The term that refers to an increase by natural growth or addition is "accretion." This word is often used in contexts such as environmental science, geology, and finance, where it describes a gradual buildup or accumulation of something over time. For instance, in geology, accretion can refer to the process by which land masses grow larger due to the accumulation of sediment, or in finance, it can describe the increase in value of an investment due to the accumulation of earned interest.

The other terms have related meanings but do not specifically convey the idea of gradual or natural growth. "Augmentation" generally refers to an increase or enhancement, often through external means or deliberate action. "Expansion" implies a broader or more generalized increase, not necessarily tied to a natural process. "Extension" suggests an elongation or spreading out of something, rather than a gradual accumulation. Thus, “accretion” is the most precise term for an increase that occurs as a result of natural growth or addition.