A PubMed Central Identification Number (PMCID) is a unique identification number assigned to papers submitted to and successfully processed by PubMed Central (PMC). PMC is a free full-text archive of biomedical and life sciences journal literature at the National Library of Medicine. It is also the designated repository for papers submitted in accordance with the NIH Public Access Policy.
Once a PMCID is assigned to a paper, in PubMed Central, it appears on the article's Abstract page.

Please keep in mind that PMC is NOT PubMed and a PMID (a unique identification number assigned by PubMed) is NOT interchangeable with a PMCID. A PMCID is required for all papers subject to the NIH Public Access Policy. A PMID is not acceptable. Learn how to get a PMCID for your paper here.