A bibliographic database is a database of bibliographic records, an organized digital collection of references to published literature, including journal and newspaper articles, conference proceedings, reports, government and legal publications, patents, books, etc.
WOS: Web of Science (previously known as Web of Knowledge) is an online subscription-based scientific citation indexing service originally produced by the Institute for Scientific Information (ISI), now maintained by Clarivate Analytics (previously the Intellectual Property and Science business of Thomson Reuters)
Scopus: IT is one of the largest abstract and citation database of peer-reviewed literature: scientific journals, books and conference proceedings
MathSciNet: The scholarly literature of mathematics is expanding at a geometric rate. It is an online publication of the American Mathematical Society, combines powerful search functionality with the authority of a team of trained mathematicians, editors and over 19,000 active reviewers. These professionals make MathSciNet the most reliable source in the field.