Object Identifiers (OIDs) are an identifier mechanism standardized by the International Telecommunications Union (ITU) for naming any object, concept, or "thing" with a globally unambiguous persistent name.
As defined in the ITU X.690 standard, the OID is primarily used in X.509 Certificates.
The OID as used in X.509 does not really apply to objects, per se, but mostly to attributes of objects. So the name is misleading to those not familiar with its usage.
The preferred term for most uses in the Document Object Identifier as defined in ISO and IETF.