M

Mass density (kg/m³)
The mass of a material or fluid per volume.

Mean radiant temperature (°C)
The area-weighted mean temperature of the surfaces surrounding a space.

Mean whole-house temperature (°C)
A volume-weighted average of room temperatures in a dwelling over a stated period (during heating only, or over the 24-hour daily cycle).

Murocaust
Similar to a hypocaust, except that a massive wall with air channels is used and the air flow is vertical instead of horizontal.