According to Wikipedia:
Open-source intelligence (OSINT) is a multi-factor (qualitative, quantitative) methodology for collecting, analyzing and making decisions about data accessible in publicly available sources to be used in an intelligence context. In the intelligence community, the term "open" refers to overt, publicly available sources (as opposed to covert or clandestine sources). OSINT under one name or another has been around for hundreds of years. With the advent of instant communications and rapid information transfer, a great deal of actionable and predictive intelligence can now be obtained from public, unclassified sources. It is not related to open-source software or collective intelligence.
OSINT is the collection and analysis of information that is gathered from public, or open sources.[1] OSINT is primarily used in national security, law enforcement, and business intelligence functions and is of value to analysts who use non-sensitive intelligence in answering classified, unclassified, or proprietary intelligence requirements across the previous intelligence disciplines.
OSINT sources can be divided up into six different categories of information flow:[2]
Media, print newspapers, magazines, radio, and television from across and between countries.
Internet, online publications, blogs, discussion groups, citizen media (i.e. – cell phone videos, and user created content), YouTube, and other social media websites (i.e. – Facebook, Twitter, Instagram, etc.). This source also outpaces a variety of other sources due to its timeliness and ease of access.
Public government data, public government reports, budgets, hearings, telephone directories, press conferences, websites, and speeches. Although this source comes from an official source they are publicly accessible and may be used openly and freely.
Professional and academic publications, information acquired from journals, conferences, symposia, academic papers, dissertations, and theses.
Commercial data, commercial imagery, financial and industrial assessments, and databases.
Grey literature, technical reports, preprints, patents, working papers, business documents, unpublished works, and newsletters.