Definition from Wiktionary, the free dictionary
Greek (h)epta- (seven), for the seventh order of 10−3. The final o conforms to the finals vowel of the SI series from micro- downwards; the letter z was added, as a start in a series of terms running backwards through the alphabet.
- In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by 10−21. Symbol: z