From the Electro-Optical Industries website:
“Effective emissivity is the ratio of the total amount of energy exiting a blackbody to that which is predicted by Planck’s law. This is the value most frequently referred to as “emissivity”.
Effective emissivity of a cavity type blackbody will normally be much higher than the surface emissivity due to the multiple energy bounces inside the body cavity.”
You can read the rest on this useful and informative webpage: www.electro-optical.com/html/bb_rad/emissivity/emisivty.asp