The term was first defined by John Harrison to specify a special type of magnesia used in reactive magnesia cements which he invented. The term caustic calcined magnesia was considered imprecise as it includes all grades of magnesia that are not dead burned or electrofused regardless of temperature of firing which is very important.
In John Harrison's original patent specification reactive magnesia was defined as "calcined at low temperatures (less than 750°C) and ground to greater than 95% passing 120 micron. Generally the lower the temperature of calcination and finer the grind, the more reactive the magnesia is and the faster it hydrates. Magnesia calcined as 650 °C passing 45 micron or less is better." For a more technical definition see Reactive Magnesia
The important property associated with reactivity is the ability to hydrate in the same rate order as hydrualic cements such as Portland cement. Otherwise dimensional distress occurs.