For some old process (such 0.18um), the transition define is: time used by the signal change from 10%*VDD to 90%*VDD (rising transition) / 90%*VDD to 10%*VDD (falling transition).
For the process now (such as 90nm), the transition define is: time used by the signal change from 30%*VDD to 70%*VDD (rising transition) / 70%*VDD to 30%*VDD (falling transition).
Note: this is not defined by DC, but defined in the cell library designer.
For the unit, it's usually "ns". There is also defined in cell library file: *.lib.