It wasn't decisive. There was still a huge Persian army that was laying waste to Greece. What it did do was convince enough Greek cities to join with Athens to resist the Persians (quite a lot of the states were actually on Persia's side).
In the original what?
Thermopylae is usually spun as a heroic last stand but it was actually a catastrophic defeat by any reasonable measure.
Again it depends on how you want to read decisive, there is an argument that Salamis was the most significant.
The original Greek.
Don't disagree about what Thermopylae was but sometimes these defeats can be used as propaganda, see Dunkirk.