Description: PubMed provides access to bibliographic information that includes MEDLINE, OLDMEDLINE, as well as:

The out-of-scope citations (e.g., articles on plate tectonics or astrophysics) from certain MEDLINE journals, primarily general science and chemistry journals, for which the life sciences articles are indexed for MEDLINE.
Citations that precede the date that a journal was selected for MEDLINE indexing.
Some additional life science journals that submit full text to PubMedCentral and receive a qualitative review by NLM.