What does FCY / F_CY stand for?
This looks like a convention used by a particular microncontroller manufacturer.  Fcy or variants thereof refer to the instruction clock frequency.  "F" means frequency, and "cy" means cycle, or instruction cycle.

Tcy likewise refers to the period of each instruction cycle, with "T" meaning time.  In this case Tcy = 1 / Fcy.

Fosc refers to the raw oscillator frequency driving the micro.  This is often divided by some integer, like 2 or 4 before becoming the instruction clock frequency.

On a old PIC 16 like the 16F877, for example, the maximum clock frequency was 20 MHz, and it took 4 clock cycles to make one instruction cycle.  If you connected a 16F877 to a 20 MHz crystal, Fosc = 20 MHz, Fcy = 5 MHz, and Tcy = 200 ns.