indicates a word or phrase being defined
Used in definitions or explanations, often equivalent to 'is a ...' or 'means ...'. Follows the term being defined.
See also: というのは
「ありがとう」とは、感謝の気持ちを表す言葉です。
'Arigatou' is a word that expresses gratitude.
used for quoting thoughts, speech, etc.
Marks quoted content, often with verbs like 言う, 思う, or 聞く. Can add contrast or emphasis to a negative statement.
See also: と
彼が来るとは思わなかった。
I didn't think he would come.
Emphatic version of the particle と meaning 'with'. Used to stress the companion or counterpart.
See also: と
彼とはもう会わない。
I won't meet with him anymore.
(not) as much as; (not) so much as
Used with a negative verb to express that something is not as much as expected or not to the degree of something else.
彼ほど親切な人とは会ったことがない。
I've never met anyone as kind as him.
the fact that; to think that; such a thing as
Expresses surprise, disbelief, anger, etc. about a situation. Can appear at the end of a sentence.
あの人がそんなことをするとは。
To think that person would do such a thing.
まさか彼が犯人だとは思わなかった。
I never thought he was the culprit.
とは is a contracted, everyday form used to introduce a definition, while というのは is more explicit and formal, often used in careful explanations.
と is the plain quoting/accompaniment particle, while とは adds emphasis, contrast, or a definitional nuance on top of that basic function.
Combination of the case particle と and the topic particle は. The contraction is a natural phonetic reduction.