The term trump card can refer to any sort of action, authority, or policy which automatically prevails over all others.
Itachi -gave- Naruto a trump card in case Sasuke became evil.
It is to use against Sasuke. Hoping that Naruto never uses it makes it a "trump card". I'm willing to bet that Itachi had a feeling that Sasuke would be easily controlled by Madara and he probably prepared Naruto just in case Sasuke ever took his eyes. He even prepared Sasuke for his encounter with Madara by implanting Amaterasu and MS. What was the point of telling Sasuke the background story of EMS? I think it was a test to see which path Sasuke took.
Amaterasu is not a trump card.
In a way, Sasuke's EMS is a trump card but Itachi's gift is even more so as it is meant to be used against only Sasuke. Naruto has the real trump card

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