とはDefinition marker
とは presents a term or idea and introduces a concise definition or explanation of what it means or what it essentially is.
How it works
The expression before とは is the item being defined. The material after it should explain that item’s meaning, nature, or scope. In English, the result often becomes “X is…,” “X means…,” or “By X, we mean….”
A general definition
共生とは、異なる人々が互いを尊重しながら暮らすことだ。
Coexistence means people with different backgrounds living together while respecting one another.
Analyze共生 is presented first, and everything after とは explains its essential nature.
Defining a term within a document
この報告書でいう「地域」とは、市内の五つの地区を指す。
In this report, “the region” refers to the five districts within the city.
AnalyzeQuotation marks help show that a particular use of the word is being defined.
Asking for a definition
自由とは何か。この問いに答えるのは簡単ではない。
What is freedom? Answering this question is not easy.
Analyzeとは何か is conventional in essays, lectures, and serious discussion.
Choosing among definition forms
とは is compact and strongly suited to formal definitions. というのは does a similar job but gives the sentence a fuller, generally more conversational opening. という意味だ explicitly says that a word, sign, or situation “means” something; with とは, the following explanation itself supplies the definition.
The same form can have a different job
When a clause is followed by とは and the speaker reacts with shock, admiration, or disbelief instead of giving a definition, it is a separate use. The following sentence is not a definition.
彼が一人で山頂まで登ったとは！
He climbed all the way to the summit alone!
AnalyzeNon-target use: this expresses surprise rather than defining anything.
More examples
再生可能エネルギーとは、自然の力を利用して得られるエネルギーである。
Renewable energy is energy obtained by using natural forces.
Analyze「暫定」とは、最終決定までの一時的な措置を表す言葉だ。
“Provisional” is a word describing a temporary measure used until a final decision is made.
Analyze信頼とは、時間をかけて築かれるものだ。
Trust is something built over time.
Analyze学ぶとは何かを、改めて考えてみたい。
I would like to reconsider what it means to learn.
Analyzeこの規則における「未成年者」とは、十八歳未満の者をいう。
Under these rules, a “minor” means a person under eighteen years of age.
Analyze先生は講義の冒頭で、文化とは何かと問いかけた。
At the beginning of the lecture, the teacher asked what culture is.
Analyzeここでいう自立とは、誰の助けも借りないことではありません。
Independence here does not mean never accepting anyone’s help.
Analyze