Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. The ordinal number matching the number one trillion in a series.
 n. One of a trillion equal parts.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 adj. The ordinal form of the number one trillion.
 n. The person or thing in the trillionth position.
 n. One of a trillion equal parts of a whole
from The Century Dictionary and Cyclopedia
 Being last in order of a series of a trillion.
 Being one of a trillion parts.
 n. One of a trillion parts; the quotient of unity divided by a trillion.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 adj. the ordinal number of one trillion in counting order
 n. one part in a trillion equal parts
Etymologies
Sorry, no etymologies found.
Examples

Five'Mathematicians Shigeru Kondo and Alexander Yee's big reveal, after building and writing code for a computer that would calculate the 10 trillionth digit of pi.
Lucy Mangan's pick of the week: The story, the stat, the quote, the tweet

We might be living in the trillionth universe or any of an infinite sequential universes.

Remember that Contador tested positive for 50 picograms (a picogram is a trillionth of a gram) of clenbuterol per milliliter of urine.

But in recent weeks trading techies have started to talk about picoseconds —one trillionth of a second.

The anomalies were fleeting, each less than a trillionth of a second.

They lasted less than a trillionth of a second, which is why no one felt the disturbance and no laboratory but mine detected it.

At the end, the mark was worth one trillionth of its original value.
In Hyperinflation's Aftermath, How Germany Went Back to Gold

Remember that Contador tested positive for 50 picograms a picogram is a trillionth of a gram of clenbuterol per milliliter of urine.

Meanwhile, the country is still bogged in wars, for the eighttrillionth year in a row, with no end in sight.

Our knowledge today allows us to push back in time to barely a trillionth of a second after the universe began.
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