Object IDentifier. A 'dot' delimited hierarchically structured node and leaf namespace. Although used in many disciplines, in system administration they are commonly used in SNMP MIBs, and for naming attributes in x.500 directories and x.509 certificate attributes.
OIDs seen by system administrators "in the wild" are typically under the IANA "Private Enterprises" hierarchy, and take the form
enterprise is an IANA-assigned Private Enterprise Number, and
<etc> is an internally defined hierarchy..
For additional information on OIDs, consult the Wikipedia article.