Power (W)
Power is defined as a rate at which a system's work is done. This entails a rate of change of energy in form, quality and/or quantity.

Heat (J)
Heat is defined as the transfer of thermal energy between two bodies with differing temperatures. Heat is not a measure of thermal energy but a description of the flow of energy. Heat transfer can occur by conduction (via molecular vibration), convection (via air flow) or electromagnetic radiation (longwave radiation). A common notation used to denote heat is Q

Heat transfer rate (W)
Heat transfer rate or heat flow rate is defined as a rate of heat flow per unit time. A common notation used to denote heat transfer rate is Q·

Heat flux (W/m2)
Heat flux is defined as a rate of heat flow per unit time per unit area. A common notation used to denote heat flux is Q" or q

Heat capacity (J/°K)
Heat capacity (or thermal capacity) is defined as a material's ability to store heat. It's the amount of heat (in joules) required to raise the material's temperature by 1 °K or 1 °C. Be aware that the heat capacity of a material can change as a function of ambient temperature.

Specific Heat or Specific Heat Capacity (J/°K/kg)
Specific heat or specific heat capacity is defined as a material's heat capacity per unit mass. Examples:   Iron = 444 J/°K/kg   Building material = 800 to 1300 J/°K/kg   Liquid oil = 2000 J/°K/kg   Liquid water = 4186 J/°K/kg

Temperature (°C or °K)
Temperature is a measure of a system's thermal energy underlying the notions of "hot" and "cold".

Thermal Resistivity (°K m/W)
Thermal resistivity is defined as a material's ability to resist heat flow.   thermal resistivity = (distance × temperature gradient)/ heat flow rate Example:   glass = 1.25 °K m/W   water = 1.67 °K m/W   expanded polystyrene = 33.33 °K m/W   air = 38.46 °K m/W   wood = 8.33 to 25 °K m/W

Thermal Resistance (°K m2/W)

Thermal resistance (also known as R-value) is a measure of the temperature difference across a unit area of a material of unit thickness when a unit of heat energy flows through it per unit time. It is determined by measuring that material's resistance to heat flow per square meter of its surface area given a temperature difference of 1 degree °C (= 1 °K) between both sides of the material. Example: If the air temperature on one side of a 10 square meter slab of expanded polystyrene (thermal resistivity = 33.33 °Km/W) 20 cm thick (the resulting R-value for the slab equals 33.33/0.2 =6.66) is at 30 °C (303.15 °K) and the air temperature on the other side of the block is at 20 °C (293.15 °K), the thermal energy that will pass through the EPS block will equal 10 * 10 / 6.66= 15.02 Joules per second or 15.02 Watts.

Thermal Conductance (W/m2/K°)
Thermal conductance is a measure of the material's ability to conduct heat and is a function of the material's thermal conductivity (the reciprocal of thermal resistivity) and thickness given a temperature difference between both sides of the material. thermal conductivity = conductivity * thickness

Thermal Mass
Thermal mass describes a material's heat capacity and conductivity. Hi heat capacity and low conductivity (high resistivity) translates into high thermal mass.

Thermal Lag
Thermal lag describes a material's thermal mass in terms of time. A material with high thermal mass (high heat capacity and low conductivity) will have a high thermal lag.

R-Value (°K m2/W)
The R-value is analogous to thermal resistance (go to definition).

U-value (W/m2/°K)
The U-value is the reciprocal of thermal resistance and is analogous to thermal conductance (go to definition). It represents the material's ability to transmit thermal energy from one side of it's slab to the other given a temperature difference between both sides of the material..

U-factor :
U-factor is analogous to U-value. See energycodes.gov for more information.

Solar Heat Gain Coefficient (SHGC)
SHGC measures a material's ability to resist solar induced thermal energy. The material is typically a window. SHGC values range from 0 (no solar heat is transmitted) to 1 (all solar heat is transmitted). See energycodes.gov for more information.