Transmission + Reflection + Absorbtion = 1
or
  Transmittance + Reflectance + Absorbtance = 1
or
  Transmissivity + Reflectivity + Absorbtivity = 1

For an opaque body, absorptivity + reflectivity = 1
For an gray body, emissivity + reflectivity = 1

Some Definitions:

Emissivity, epsilon: a unitless number between 0 and 1 which defines howwell a surface emits radiation, also called emittance. This value can be different for each wavelength. The tabulated values are usually averaged over the infrared wavelengths. A perfect emitter is called a black body and has a value of 1.

Absorptivity, alpha: also called absorptance, basically the same as
emissivity, however, the tabulated values are averaged over the solar
spectrum. The value represents how well a surface absorbs radiation.

Reflectivity, rho: also called reflectance. A measure of how well a surface reflects radiation. Tabulated values are also averaged in general. A value of 1 is a perfect reflector.

Transmissivity, tau: also called transmittance. A measure of how well
radiation passes through a material. Tabulated values are also averaged in general. A value of 1 is a perfect transmitter.

The previous 3 values alpha (or epsilon), rho, and tau must add up to 1 for any given wavelength.