In units of measurement, a factor of 1018.
Based on the Greek prefix hexa‑, six, by deleting the first letter.
This is one of the standard SI (Système International) decimal prefixes (see the entry Number words). This form is rarely met with in general scientific usage; an example is exajoule. It is occasionally used in discussions predicting very fast computers in the future, as in exaflop (see ‑flop), or large amounts of data: exabyte.