What is the symbol of a diode?

The letter used is called a Reference Designation, and for a diode sometimes depends on what kind of diode it is. Most diodes use the letter D, as in D1, D2, D3. Zener diodes usually use the letter Z or much more rarely ZR or ZD (Zener Rectifier or Zener Diode). CR is an archaic designation that stands for Crystal Rectifier.

LEDs may use LED1, LED2, etc. or just D1, D2, etc.

A bridge rectifier (four diodes connected so AC fed into two leads results in pulsating DC from two other leads) uses BR for obvious reasons.

AE7HD

The letter symbol for the diode is D.