| Metabolism | (Environmental Engineering) The processes which sustain an organism, including energy production, synthesis of proteins for repair and replication. |
| Metalimnion | (Environmental Engineering) The middle layer of a lake. |
| Mixed liquor suspended solids (MLSS) | (Environmental Engineering) The total suspended solids concentration in the activated sludge tank. |
| Mixed liquor volatile suspended solids (MLVSS) | (Environmental Engineering) The volatile suspended solids concentration in the activated sludge tank. |
| Maintainability - | (Software Engineering) the degree to which a program is amenable to change |
| Maintenance - | (Software Engineering) the activities associated with changes to software after it has been delivered to end-users |
| Make-buy decision - | (Software Engineering) determining whether software should be built internally, acquired, contracted or built from reusable components |
| Measurement - | (Software Engineering) collecting quantitative data about the software or the software engineering processMetrics - |
| Milestones - | (Software Engineering) a point in time that is used to indicate progress during a project |
| Modular design - | (Software Engineering) a design approach that stresses modularity |