Dimensionally Transcendental means something that is bigger on the inside.
The Fourth Doctor explains to his companion leela in episode one of the Robots of Death that the changing dimensions are like that of two different sizes of boxes. He explains that if the larger box is placed further away from the smaller box, it looks smaller and if you can keep that looking the same size, the bigger can fit inside the smaller... but they must exaclty the same distance away. He then goes on to say that 'It was a fete of Trans-dimensional engineering' probably coming from Rassilon or Omega.