Short Answer
An ORCID is a persistent digital identifier (Open Researcher and Contributor ID) to uniquely identify scientific and other academic authors and contributors.
Detailed Answer
An ORCID (Open Researcher and Contributor ID) is the common name for the ORCID iD, a persistent digital identifier that is issued to scientific and other academic authors and contributors upon registration. The ORCID allows to uniquely identify individual authors and link any scientific and academic publications and contributions to their profile.
ORCID is also the name of a non-profit organization that provides ORCIDs along services for this identifier, learn more at the official ORCID webpage and the wikipedia entry for ORCID.