I think it means Timer Oscillator.
For example on a PIC, there will be 2 pins TOSC1 and TOSC2 which can be wired to a crystal oscillator. This is a sort of secondary oscillator which runs a timer on the chip. This is useful for having a Real Time Clock in the pic since a crystal with frequency 32768 KHz can be used (has nice divisibility).
Oh and Tosc can also be the time for one pulse from the oscillator. For a PIC, for every 4 oscillator pulses, one cycle happens. So Tcycle = 4*Tosc.