twentieth letter of the English alphabet; in the Phoenician alphabet the corresponding sign was the 22nd and last; everything after T in the modern alphabet represents European alterations or additions. The sound has been consistent throughout its history.

In Late Latin and Old French, -t- before -e- and -i- acquired the "s" value of -c- and words appeared in both spellings (nationem/nacionem) and often passed into Middle English with a -c- (nacioun). In most of these the spelling was restored to a -t- by or in the period of early Modern English, but sorting them out took time (Edmund Coote's "English Schoole-maister" (1596) noted malicious/malitious) and a few (space, place, coercion, suspicion) resisted the restoration. 

To cross one's t's(and dot one's i's) "to be exact" is attested from 1849. Phrase to a T "exactly, with utmost exactness" is recorded from 1690s, though the exact signification remains uncertain despite much speculation. The measuring tool called a T-square (sometimes suggested as the source of this) is recorded by that name only from 1785. The T-cell (1970) so called because they are derived from the thymus. As a medieval numeral, T represented 160. A T was formerly branded on the hand of a convicted thief.

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Definitions of T from WordNet

t (n.)
a base found in DNA (but not in RNA) and derived from pyrimidine; pairs with adenine;
Synonyms: thymine
t (n.)
one of the four nucleotides used in building DNA; all four nucleotides have a common phosphate group and a sugar (ribose);
Synonyms: deoxythymidine monophosphate
t (n.)
a unit of weight equivalent to 1000 kilograms;
Synonyms: metric ton / mt / tonne
t (n.)
thyroid hormone similar to thyroxine but with one less iodine atom per molecule and produced in smaller quantity; exerts the same biological effects as thyroxine but is more potent and briefer;
Synonyms: triiodothyronine / liothyronine
t (n.)
hormone produced by the thyroid glands to regulate metabolism by controlling the rate of oxidation in cells;
Synonyms: thyroxine / thyroxin / tetraiodothyronine