This thing: https://material.io/components/text-fields/

I want a concise definition, what feature makes it so? If I can't tell what is and what isn't a thingy the whole article is pointless.

Ironically, that's exactly what the article is about- In MD users can't tell what isn't nor isn't a thingy.

Anyway, it's like crystal meth. If you don't know what it is you are winning! The advice is for people who are unlucky enough to be near it.

A text field following the Material Design specification.

If you don't know what it is, you probably aren't going to accidentally use them, and aren't a target of the advice, since you would probably have to either read the spec or adopt a library that waves around being an implementation of the spec to do it.

The term is well-known and understood by the intended audience.

The article links to Material Design in literally its first line!