A t-unit was defined as a unit of discourse capable of functioning as a sentence.
A t-unit is the main clause of a sentence plus any other subordinate clauses that are attached to it. So, for example, the sentence Bill likes to run contains one t-unit, and the sentence Bill likes to run with his friends contains two t-units.
Applying an index of linguistic development in EFL classrooms
http://www.jalt-publications.org/tlt/articles/2004/05/jenks
DISCOURSE FUNCTIONS AND SYNTACTIC COMPLEXITY IN SYNCHRONOUS AND ASYNCHRONOUS COMMUNICATION
http://llt.msu.edu/vol4num1/sotillo/default.html