Deciphering the identifier for a diode
This is a suppressor diode (TVS suppressor).

600W, 16V, unidirectional (yours is "A"), for bidirectional it would be "CA".

EDIT:

Lets just say it is a powerful zener diode. 

Yours should read 0.7V in the forward direction, and should start clamping at around 13.xxV in the other direction. You can test the 0.7V with a mutimeter diode tester, the 13.xx V you can test with a power supply of about 18V minimum and a series resistor, say 100 Ohm if you use a 18V power supply.