Stunts are totally different between ICONS and FATE.
Use the definitions found in the ICONS book (since the FATE system’s version of Stunts are not used in ICONS).
In ICONS a STUNT is called a POWER STUNT.
A Power Stunt is when a character spends a Determination Point to use a Power they don’t normally have but seems logical that they could use from time to time (i.e. “I didn’t know you could do that!”).