What is a small or imperceptible addition or augmentation called?

The term "increment" refers to a small or gradual increase, which aligns perfectly with the description provided in the question. In many contexts, increments can describe tiny additions in measurements, numerical values, or even conceptual frameworks, emphasizing the gradual nature of change or enhancement. This term is commonly used in fields such as mathematics, finance, and computer programming, where small changes can lead to significant outcomes over time.

In contrast, the other terms don't fit the description: "incidence" pertains to the occurrence of events, "incumbent" refers to someone holding a particular position or role, and "incrimination" relates to accusing someone of wrongdoing. None of these choices convey the idea of a small addition or growth as effectively as "increment."