A DOI is a Digital Object Identifer. It is a unique identifying number/charactoer string for an electronic article, ebook, or other "object". It provides a permanent, stable access to these objects on the internet.
All DOI numbers begin with a 10, such as: 10.1159/000330840 or 10.1289/ehp.1003206.
Several citations styles now incorporate DOIs for electronic sources when present. Not all articles have DOIs.
If you are unable to locate the DOI for a source, try Crossref
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