B

Step 1: Understanding the definition: The emissive power of a body refers to its ability to emit radiation. A black body, in contrast, is an idealized physical object that perfectly absorbs all incident electromagnetic radiation and re-emits it as thermal radiation. The ratio of the emissive power of a real body to that of a black body at the same temperature is crucial in thermodynamics and heat transfer.
Step 2: Emissivity is a dimensionless quantity that defines this ratio. It is given as:
$$ \epsilon = \frac{E}{E_b} $$
where $E$ is the emissive power of the body and $E_b$ is the emissive power of a black body.
Step 3: Therefore, the term that describes the ratio of emissive power of a body to that of a black body is known as emissivity.
Conclusion: Hence, the correct option is B: Emissivity.