PMID (PubMed ID)
The unique identifier PubMed (the National Library of Medicine's biomedical literature database) assigns to an indexed study. Every citation with a PMID on this site can be looked up directly at pubmed.ncbi.nlm.nih.gov/<PMID>/ to read the original study.
Example
Ferracioli-Oda et al.'s melatonin meta-analysis carries PMID 23691095.