A series of equal deposits or payments is referred to as a(n):

A series of equal deposits or payments is referred to as an annuity because this term specifically describes a financial product or arrangement where payments are made at regular intervals over a specified period. Annuities can take various forms, such as regular withdrawals from a retirement account or structured payments made towards a loan. They are commonly utilized to secure a steady income stream, often during retirement.

In contrast, the other options do not accurately define a series of equal payments. An investment account pertains to a financial account that holds assets like stocks and bonds, but does not imply regular payments. A loan agreement is a contract where funds are borrowed, which may involve varying payment amounts rather than a fixed series. A dividend plan usually refers to the distribution of profits to shareholders, reflecting corporate earnings rather than regular payments made by an individual or entity. Thus, the characteristics of annuities distinctly set them apart from these other financial concepts.