How to interprete this と? [duplicate]

Question

The following sentence stands isolated:

世界遺産とは世界の子供たちや孫たちに残していかなければならない自然や建物のことである。

First, my attempt at translation: "With the world heritage, there are buildings and nature which we must from now on leave to our children and grand children."

I've no idea how to meaningfully translate this とは. と "adds up" with は here, but I can't make much sense of と in this position.

I must add that I feel very unsure about the whole sentence. I just learned about the -ていく construction which expresses that one is entering times of change from now on. I made 残していかなければならない an attribute to 自然や建物 because I couldn't muster an interpretation which would have made more sense. But I couldn't find a way to translate 残していかなければならない without 自然や建物 and this makes me doubt my understanding of the -ていく pattern...^^

Answer 1

The とは is commonly used to express an explanatory or defining "is a".

It is not uncommon to find text (in forums or sites) asking

〜とは？

to ask people what something is. Think of it as used in definitions.

Q: 学校とは？ What is a school?

A: 人が教育を受ける場所である。 It is where people receive education.

Answer 2

You can think of  とは as a formal version of は.

As for ~ていく, you can think of it as "going in the direction of that verb".

So here it's more like "we must go in the direction of leaving nature and buildings for our posterity". The ていく is being used to express movement, or change as you stated.