The Collaborative International Dictionary of English v.0.48:

Emissivity \Em`is*siv"i*ty\, n.
   Tendency to emission; comparative facility of emission, or
   rate at which emission takes place; specif. (Physics), the
   rate of emission of heat from a bounding surface per degree
   of temperature difference between the surface and surrounding
   substances (called by Fourier external conductivity).
   [Webster 1913 Suppl.]