Yotta is the third largest decimal unit prefix in the metric system, denoting a factor of 10²⁴ or 1000000000000000000000000; that is, one million million million million, or one septillion. It has the unit symbol Y. The prefix name is derived from the Ancient Greek οκτώ (októ), meaning "eight", because it is equal to 1,000⁸. It was added as an SI prefix to the International System of Units (SI) in 1991.

Usage examples:

• The mass of the Earth is 5,972.6 Yg.
• The mass of the oceans is about 1.4 Yg.
• The total power output of the Sun is approximately 385 YW.
• The observable universe is estimated to be 880 Ym in diameter.
• One yottabyte (YB) is a unit of digital information or information storage capacity that contains one septillion bytes or 1,000 zettabytes. The yobibyte (YiB) is a related unit that uses a binary prefix, and means 1,024⁸ bytes, which is approximately 1.2 septillion bytes.