teras- (symbol: T) is a prefix in the SI system of units denoting 1012, or 1,000,000,000,000 (1 trillion).

Confirmed in 1960, it comes from the Greek τέρας, meaning monster. It also bears a resemblance to the Greek prefix τετρα- meaning four; the coincidence of it signifying the fourth power of 1000 served as a model for the higher-order prefixes peta-, exa-, zetta- and yotta-, all of which are deliberately distorted forms of the Latin or Greek roots for the corresponding powers (fifth to eighth respectively) of 1,000.

In computer science tera- can sometimes mean 1,099,511,627,776 (240) instead of 1,000,000,000,000, especially in the term terabyte. To avoid this ambiguity, the binary prefix tebi- has been introduced to signify 240, but this, in common with the other binary prefixes, is not currently in general use.


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