VCO IEEE paper revue
It's the frequency, where the intrinsic gain curve of a device (means: isolated device, no load, voltage gain-vs-frequency-decay = -20dB/decade) hits the 0db-line (vu=1). At this transit frequency (fT) the voltage gain by definition is =1, but the power gain may still be >1, so it is possible to use it as an oscillator or even as an (power) amplifier still beyond this fT.
It's also a clue that you can achieve a voltage gain of 20dB @ fT/10 with this device and an appropriate circuit.