Basically I'm wondering if there is a concise way to represent the loss of understanding a foreigner would experience if they were to encounter an American idiom for the first time.
An example: http://www.youtube.com/watch?v=gjwofYhUJEM
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As far as the question in the title of your question, I believe the word you are looking for is idiom.
And what the foreigner would experience, your word is likely confusion.