PubMed.

https://www.ncbi.nlm.nih.gov/pubmed/

"PubMed comprises over 26 million citations for biomedical literature from MEDLINE, life science journals, and online books. PubMed citations and abstracts include the fields of biomedicine and health, covering portions of the life sciences, behavioral sciences, chemical sciences, and bioengineering. PubMed also provides access to additional relevant web sites and links to the other NCBI molecular biology resources.

"PubMed is a free resource that is developed and maintained by the National Center for Biotechnology Information (NCBI), at the U.S. National Library of Medicine (NLM), located at the National Institutes of Health (NIH)" https://www.ncbi.nlm.nih.gov/books/NBK3827/#pubmedhelp.PubMed_Quick_Start, accessed 12-2016.

From the Wikipedia article on PubMed, accessed 12-2016:

"PubMed is a free search engine accessing primarily the MEDLINE database of references and abstracts on life sciences and biomedical topics. The United States National Library of Medicine (NLM) at the National Institutes of Health maintains the database as part of the Entrez system of information retrieval.

From 1971 to 1997, MEDLINE online access to the MEDLARS Online computerized database had been primarily through institutional facilities, such as university libraries. PubMed, first released in January 1996, ushered in the era of private, free, home- and office-based MEDLINE searching.^[1] The PubMed system was offered free to the public in June 1997, when MEDLINE searches via the Web were demonstrated, in a ceremony, by Vice President Al Gore.^[2

In addition to MEDLINE, PubMed provides access to:

• older references from the print version of Index Medicus back to 1951 and earlier;
• references to some journals before they were indexed in Index Medicus and MEDLINE, for instance Science, BMJ, and Annals of Surgery;
• very recent entries to records for an article before it is indexed with Medical Subject Headings (MeSH) and added to MEDLINE; and
• a collection of books available full-text and other subsets of NLM records.^[3]
• PMC citations

Many PubMed records contain links to full text articles, some of which are freely available, often in PubMed Central^[4] and local mirrors such as UK PubMed Central.^[5]

Information about the journals indexed in MEDLINE and available through PubMed is found in the NLM Catalog.^[6]