What characteristic defines the series of payments provided by an annuity?

An annuity is defined by its structure of payments, which are made at regular, equal intervals. This characteristic is integral to the concept of annuities, as it differentiates them from other types of financial products. The regularity of these payments means that individuals can rely on a predictable income stream over a specified period, making them a popular choice for retirement planning or structured settlements.

In contrast, the other options describe characteristics that are not typical of standard annuity arrangements. For instance, payments that vary over time based on market performance pertain more to investment products rather than fixed annuities, which specifically aim for stability and consistency. Payments made at variable intervals would also not provide the certainty that annuity holders seek. Lastly, while some financial products may offer indefinite payments, standard annuities are typically structured around a specific term, either for a number of years or until the death of the annuitant, rather than an unscheduled connotation of indefinite payments. Thus, the defining trait of regular, equal payments is what solidifies option C as the correct answer.