とは

とは after a plain-form clause means "to think that…": the speaker had not expected this at all. A noun or な-adjective takes it with or without だ. The reaction often follows – おもわなかった思わなかった, しらなかった知らなかった, おどろいた驚いた – or the sentence stops at とは.

まさか at the start of the clause makes the surprise stronger, and an evaluation can replace the reaction: すごい, しんじられない信じられない, たいした大したものだ.

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This とは is written and dramatic: narration, essays, a character on stage. In conversation the same surprise is なんて, which attaches in the same place.

Defining a word

After a noun, とは introduces what the word means: "X is…", "what is X?". The sentence often ends in のことだ, といういみ意味だ or ものだ. This use is not about surprise at all, and it is common in textbooks and dictionaries.