Exa is a prefix in the International System of Units (SI) that denotes a factor of 10^18 or one quintillion. It is often used in scientific and technical contexts to express large quantities, such as data storage or computing power. For example, one exabyte is equal to one quintillion bytes. The term "exa" is derived from the Greek word "hex," meaning six, as it represents 2^60 in binary terms. This prefix is part of a series of metric prefixes that include kilo, mega, giga, and tera, which are used to simplify the representation of large numbers in various fields, including computer science and physics.