What does the metric designation M10 indicate about a fastener?

The metric designation M10 signifies that the fastener has a nominal thread diameter of 10 mm. In metric fastener nomenclature, the 'M' indicates that it is a metric thread, and the number that follows specifies the diameter of the thread in millimeters. Therefore, when you see M10, it's understood that the diameter is precisely 10 mm. This system allows for a straightforward way to identify the size of a fastener, facilitating easier selection and compatibility with other components in mechanical applications.