McDonald's is the brand name shortened to the nickname “Mickey D's” in North America.
The nickname is an informal pronunciation of the company name, not an official rebranding. It became especially associated with regional speech in the United States and Canada, where customers used it as a casual alternative to saying McDonald's in full.
The official name comes from the surname of brothers Richard and Maurice McDonald, whose California restaurant business inspired the later chain. Ray Kroc expanded the system nationally after becoming involved with the brothers' operation.
“Mickey D's” should not be confused with McDonaldland, the fictional advertising world, or the Golden Arches, the company's visual symbol. Those are separate brand elements. The nickname refers directly to the company and restaurant chain itself.