Turn-on voltage refers to the forward voltage of a semiconductor diode. It is the required voltage across the diode before it starts to conduct current in the forward direction. For silicon diodes, the turn-on voltage is approximately 0.7V, while for germanium diodes is approximately 0.3V.
The voltage required to bring the diode into significant conduction.
Microelectronic Circuit Design, 4th Edition by Richard C. Jaeger & Travis N. Blalock