Used in the metric [decimal] system as one trillion [U.S.] and billionfold [U.K.]: 10¹² [1 000 000 000 000] or a million million.

The metric symbol for tera- is T.

According to the Academic Press Dictionary of Science and Technology, Harcourt Brace Jovanovich, Publishers, 1992: "Tera- comes "from a Greek term meaning ‘marvel' or 'monster,' with the sense that this is a huge and marvelous quantity."

It would make much more sense to have this metric unit (tera) based on tetra- from Greek for "four" then to have terato-, "monster"; which has no relative etymological connection with this or any metric unit.