What means ambient in TypeScript

Question

I don't understand what means the word ambient in the following sentence:

A function implementation cannot be declared in an ambient context.

I'm not sure to understand the general meaning of the word, (English isn't my maternal language) and if there is a specific meaning here I don't get it as well.

I've tried to understand in my maternal language but couldn't get it in this context. It's something like current context I'd say but it doesn't work out.

The message appeared because I was trying to declare a class, which cannot be declared, only module can. I've fixed it but still don't understand the meaning of the error message here.