Material outside exclusive protection
Works commonly enter the public domain when their economic copyright term expires. Some subject matter was never eligible: the U.S. Copyright Office identifies facts and discoveries as examples of that boundary.
The term describes a rights status rather than a universal catalogue. Work categories, historical formalities, dates and territorial rules can affect the result. Expiry in one jurisdiction should not simply be assumed everywhere else.
Public access differs from public-domain status
A website or digital collection can provide access to protected works. Online availability, a download button or free viewing does not establish that economic rights have expired.
An open licence instead grants permissions while rights continue to exist, potentially with attribution or other conditions. Reliance on public-domain status and compliance with an active licence are therefore different bases for use.
References: [1]
Distinguishing underlying material and new expression
An old work and a newly authored translation or other original expression require separate identification. Unprotected facts or ideas do not automatically make every new passage discussing them unprotected; the material actually being used matters.
Public-domain analysis here concerns copyright. WIPO frames its explanation in terms of economic rights, rather than a general exemption from every legal rule or authorship-related interest.