Transwarp threshold
The transwarp threshold (also known as the maximum warp barrier or the transwarp barrier) was a boundary that separated warp from transwarp and was identified in the 24th century as the velocity of warp 10. 

In 2372, Tom Paris became the first Human to cross the transwarp threshold in the Class 2 shuttle . While the attempt was successful, the Federation transwarp drive turned out to be an unsafe method of bringing  back to the Alpha Quadrant, as it induced celerity-induced accelerated somatic mutation rate in Humans. ()

Ensign Brad Boimler mentioned Paris's achievement to Ensign Beckett Mariner on the  in 2381. ()

See also
Time barrier
Warp 2 barrier

fr:Seuil de transdistorsion
Threshold