The term initially refers to the extraction of information from publicly available sources: Newspapers and magazines (print and online), social media, company information subject to disclosure obligations. Open Source Intelligence (OSINT) originates in the realm of intelligence services. But of course, this method also plays a role for everyone who needs to assess a person or a partner company.
For those who like to read political thrillers, I can recommend the book "True Faith and Allegiance". This pageturner is part of the Tom Clancy series and was written by Mark Greaney. The thriller describes a scenario around international terrorism, where publicly available information becomes the Achilles' heel of open US society. The book is extraordinarily well researched, has a great suspense and surprises again and again with suspenseful twists and turns.