PMID is an acronym for PubMed Identifier or more specifically PubMed Unique Identifier which is a unique number assigned to each PubMed citation of life sciences and biomedical scientific journal articles. It is used similarly to the International Standard Book Number (ISBN) for books.
As of 2005, there are roughly between 15-16 million numbers in use, starting from PMID 1, and about 1 million new numbers are added each year. Unique Identifier * (PubMed Unique Identifier PMID) is the search field tag used in the PubMed search query.
By using the PMID as a search argument (with of without the * field tag), the relevant abstract will be displayed by PubMed. Multiple PMIDs in one search will yield all abstracts (it is interpreted as an OR operation). PMID itself should not be included in the search.
To search in combination with other terms, one must enter the search field tag, e.g., smith AND (10403340 *).