The Collaborative International Dictionary of English v.0.48:

Trillion \Tril"lion\, n. [F. trillion, formed from the pref.
   tri- in imitation of million a million. Cf. Billion.]
   According to the French notation, which is used upon the
   Continent generally and in the United States, the number
   expressed by a unit with twelve ciphers annexed; a million
   millions; according to the English notation, the number
   produced by involving a million to the third power, or the
   number represented by a unit with eighteen ciphers annexed.
   See the Note under Numeration.
   [1913 Webster]

WordNet (r) 3.0 (2006):

trillion
    adj 1: one quintillion in Great Britain
    2: one million million in the United States
    n 1: a very large indefinite number (usually hyperbole); "there
         were millions of flies" [syn: million, billion,
         trillion, zillion, jillion, gazillion]
    2: the number that is represented as a one followed by 18 zeros;
       "in England they call a quintillion a trillion" [syn:
       trillion, one million million million]
    3: the number that is represented as a one followed by 12 zeros;
       "in England they call a trillion a billion" [syn: trillion,
       one million million, 1000000000000]

The Free On-line Dictionary of Computing (30 December 2018):

trillion

    In Britain, France, and Germany, 10^18 or a
   million cubed.

   In the USA and Canada, 10^12.

   [Elsewhere?]

   (1996-10-03)