PubMed

PubMed is a free database accessing primarily the MEDLINE database of references and abstracts on life sciences and biomedical topics. The database is maintained by the United States National Library of Medicine at the National Institutes of Health as part of the Entrez system of information retrieval.

PubMed identifier
The PubMed identifier, whose acronym is PMID is a unique number assigned to each PubMed record that not change over time or during processing and are never reused.