とは

とは • (to wa) 

1. indicates that a word or a phrase is to be defined

   素(そ)数(すう)とは何(なに)かを学(まな)んだ。

   Sosū to wa nani ka o mananda.

   I learnt what a prime number was.

   「原(げん)子(し)力(りょく)」とは？

   “Genshiryoku” to wa?

   What is "nuclear power"?

2. (more emphatic) Synonym of と (to, “with”)

   あいつとはもう関(かん)係(けい)ない。

   Aitsu to wa mō kankei nai.

   I have nothing to do with them anymore.

   過(か)去(こ)とは違(ちが)う自(じ)分(ぶん)

   kako to wa chigau jibun

   a self who is different from the [self of the] past

3. Short for とは思わなかった (to wa omowanakatta, “did not think that ...”).

   まさか自(じ)殺(さつ)するとは……

   Masaka jisatsu suru to wa……

   I never would have thought that he/she would kill himself/herself...

• This particle is often used, for example, in encyclopedias to define entries.
• When used alone with a word it denotes the meaning of "this word is defined as?" and may be translated as "about" or "What is".

• 就(つ)いて (tsuite): regarding, concerning