What term is used to describe an excess or superabundance of something?

The term that best describes an excess or superabundance of something is "plethora." This word specifically conveys the idea of a large quantity and often a sense of excess, indicating that there is not just a significant amount but potentially more than is needed or normal.

While "surplus" also refers to an excess, it typically implies that what is being discussed is more than what is required or that it exceeds the expected amount. "Overload" generally refers to too much of something being put onto a system or individual, resulting in strain or difficulty, rather than just a simple excess in quantity. "Abundance" indicates a large quantity as well, but it usually does not carry the same connotation of excess as "plethora" does. Therefore, "plethora" is the most accurate choice for describing an overwhelming or excessive amount.