stands for USB 2.0 Transceiver Macrocell Interface.
The UTMI defines the interface with which vendor ASIC implementations of USB 2.0 protocol use the USB Transceiver Macroscell or UTM. The UTM is responsible for all the physical (low) level communication between USB devices: signalling, synchronization, data serialization, bit stuffing, clock recovery.
But the main use the UTM was designed for is to divide clock domains of the UTM and the USB controller logic. The thing is that the new USB 2.0 standard operates at the very high rate of 480 Megabits/sec (that's 40 times faster than the USB 1.1 standard) which stands for 480Mhz clock. These kind of clocks are hard to achieve on chip and using the UTM allows using clocks slower by factor of 8, that is 60Mhz which is most acceptable for today ASIC technology.
UTMI version 2.0 transceivers are also backward compatible with the old (or should I say 'current') USB 1.1 technology which operates at the 12Mbits/sec rate.