**blackbody** |
A body with a surface emissivity of 1. Such a body will emit all of the thermal radiation it can (as described by theory), and will absorb 100% of the thermal radiation striking it. Most physical objects have surface emissivities less than 1 and hence do not have blackbody surface properties. |

**density, r** |
The amount of mass per unit volume. In heat transfer problems, the density works with the specific heat to determine how much energy a body can store per unit increase in temperature. Its units are kg/m^{3}. |

**emissive power** |
The heat per unit time (and per unit area) emitted by an object. For a blackbody, this is given by the Stefan-Boltzmann relation s**T*^{4} |

**graybody** |
A body that emits only a fraction of the thermal energy emitted by an equivalent blackbody. By definition, a graybody has a surface emissitivy less than 1, and a surface reflectivity greater than zero. |

**heat flux, ***q* |
The rate of heat flowing past a reference datum. Its units are W/m^{2}. |

**internal energy, ***e* |
A measure of the internal energy stored within a material per unit volume. For most heat transfer problems, this energy consists just of thermal energy. The amount of thermal energy stored in a body is manifested by its temperature. |

**radiation view** factor, *F*_{12} |
The fraction of thermal energy leaving the surface of object 1 and reaching the surface of object 2, determined entirely from geometrical considerations. Stated in other words, *F*_{12} is the fraction of object 2 visible from the surface of object 1, and ranges from zero to 1. This quantity is also known as the Radiation Shape Factor. Its units are dimensionless. |

**rate of heat** generation, *q*_{gen} |
A function of position that describes the rate of heat generation within a body. Typically, this new heat must be conducted to the body boundaries and removed via convection and/or radiation heat transfer. Its units are W/m^{3}. |

**specific heat, ***c* |
A material property that indicates the amount of energy a body stores for each degree increase in temperature, on a per unit mass basis. Its units are J/kg-K. |

**Stefan-Boltzmann** constant, s |
Constant of proportionality used in radiation heat transfer, whose value is 5.669 x 10^{-8} W/m^{2}-K^{4}. For a blackbody, the heat flux emitted is given by the product of s and the absolute temperature to the fourth power. |

**surface emissitivy, e** |
The relative emissive power of a body compared to that of an ideal blackbody. In other words, the fraction of thermal radiation emitted compared to the amount emitted if the body were a blackbody. By definition, a blackbody has a surface emissivity of 1. The emissivity is also equal to the absorption coefficient, or the fraction of any thermal energy incident on a body that is absorbed. |

**thermal conductivity, ***k* |
A material property that describes the rate at which heat flows within a body for a given temperature difference. Its units are W/m-k. |

**thermal diffusivity, a** |
A material property that describes the rate at which heat diffuses through a body. It is a function of the body's thermal conductivity and its specific heat. A high thermal conductivity will increase the body's thermal diffusivity, as heat will be able to conduct across the body quickly. Conversely, a high specific heat will lower the body's thermal diffusivity, since heat is preferentially stored as internal energy within the body instead of being conducted through it. Its units are m^{2}/s. |