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 Words for multiples). 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.