Identify the prefix that indicates a factor of 10^-18.

The prefix that indicates a factor of 10^-18 is "atto." This prefix is derived from the Danish word "at," meaning "at" or "to," and it represents an extremely small quantity in the International System of Units (SI). When you encounter measurements or calculations that require representation of such small values, "atto" denotes one quintillionth, or 0.000000000000000001.

In the context of the prefix system, "femto" represents a factor of 10^-15, "zepto" corresponds to 10^-21, and "quepto" is not an officially recognized SI prefix at this time. Therefore, "atto" is the correct choice for a factor of 10^-18, as it fits perfectly within the established hierarchy of metric prefixes that denote decreasing powers of ten.