| M/SCI | Mission/Safety Critical Item 
| Others |
| MSCI | Management Solutions Consulting, Inc.
| Others |
| MSCI | Margaret Sanger Center International
| Others |
| MSCI | Maruti Suzuki Club India
| Others |
| MSci | Master in Science (Master of Natural Sciences at Cambridge)
| Others |
| MSCI | Master of Science in Clinical Investigation
| Others |
| MSCI | Mathematical Sciences Career Information
| Others |
| MSCI | Medical Service Corporation International
| Others |
| MSCI | Meiotic Sex Chromosome Inactivation
| Others |
| MSCI | Metal Service Center Institute
| Others |
| MSCI | Metals Service Center Institute
| Others |
| MSCI | Michigan Speech Coaches, Incorporated
| Others |
| MSCI | Microsat Systems Canada Incorporated
| Others |
| MSCI | Minnesota School of Computer Imaging
| Others |
| MSCI | Minnesota Supercomputer Center, Incorporated
| Others |
| MSCI | Mission Scientist
| Others |
| MSCI | Mississippi Central Railroad Company
| Others |
| MSCI | Mississippi Space Commerce Initiative
| Others |
| MSCI | Morgan Stanley Capital Index
| Others |
| MSCI | Morgan Stanley Composite Index
| Others |