What is Cardboard?

Legal Definition
Cardboard is a generic term for a heavy-duty paper of various strengths, ranging from a simple arrangement of a single thick sheet of paper to complex configurations featuring multiple corrugated and uncorrugated layers.

Despite widespread use in general English and French, the term is deprecated in business and industry. Material producers, container manufacturers, packaging engineers, and standards organizations, try to use more specific terminology. There is still no complete and uniform usage. Often the term "cardboard" is avoided because it does not define any particular material.
-- Wikipedia
Legal Definition
(of a character in a literary work) Lacking depth and realism; artificial.