1 a million million (1,000,000,000,000 or 10(12)).
2 (formerly, esp. Brit.) a million million million (1,000,000,000,000,000,000 or 10(18)).
3 (in pl.) colloq. a very large number (trillions of times).
Usage:
Senses 1-2 correspond to the change in sense of billion.
Derivatives:
trillionth adj. & n.
Etymology: F trillion or It. trilione (as TRI-, MILLION), after billion
Useful english dictionary. 2012.