Thermal conductivity of a material may be defined as the

• quantity of heat flowing in one second through a slab of the material of area 1 cm2 and thickness 1 cm when its faces differ in temperature by 1° C
• quantity of heat flowing in one second through one cm of material when opposite faces are maintained at a temperature difference of 1° C
• heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces
• all of the above