Manometer - A U-shaped device used to measure pressure difference.

Matter - Anything with mass is known as matter. Lead, salt, water, alcohol are all considered matter. All these forms represent the three basic forms of matter. Solid, Liquid, Gas.

Mass - A measure of an object's resistance to acceleration; different from but proportional to the object's weight.

Mechanical Cooling - Conventional cooling provided by a mechanical means. Term can be interchanged with "active cooling".

Mechanical Efficiency - The ratio of work done by a machine compared to the amount of energy used.

Megohm - One million ohms.

Methyl Alcohol - is sometimes used as an antifreeze. 

Methyl Chloride - One type of Refrigerant.

Metering Device- A device used to meter the amount of refrigerant entering the evaporator.

Moeller diagrams - A graphical representation of the thermal properties of fluids with total heat and entropy as coordinates.

Motor - A device used to convert electrical energy into mechanical energy.