A black body is a body that absorbs all the Electromagnetic radiation incident on it. To maintain thermal equilibrium, radiation must be emitted at the same rate it is being absorbed, so a black body radiates as well.
Black body radiation is defined as the thermalelectromagnetic radiation inside or surrounding a body which is in a state of thermodynamic equilibrium with the surrounding environment, or which is emitted by any black body ( defined as a non-reflective, idealized and opaque body). It has a precise intensity and spectrum -that depends merely on the temperature of the body- which is assumed to be uniform and constant.
The thermal radiation naturally emitted by normal objects can be expressed as black body radiation. An enclosure which is perfectly insulated that is in thermal equilibrium, contains black body radiation internally, and will emit this radiation through a hole made in its wall, if it is small enough to have negligible effect upon the equilibrium.
What makes a black body
A black body appears black at room temperature, as most of the energy it radiates cannot be observed by the human eye as it is infra-red. Because light waves at lower frequencies can't be observed by the human eye, a black body appears grey when viewed in the dark at the lowest visible temperature, despite the infrared range being its objective physical spectrum peak.
Although planets and stars are neither perfect black bodies nor in thermal equilibrium with their surroundings, black body radiation is used as a first approximation for the amount of energy they emit.
Black holes are nearly perfect black bodies, as they absorb all the radiation falling on them. It has been proposed that black holes emit black body radiation (named Hawking radiation), with a temperature that relies on the black hole mass.
Gustav Kirchhoff introduced the term black body in 1860. Black body radiation is also named cavity radiation, thermal radiation, complete radiation and temperature radiation.
If an object is heated up to around 1500 degrees a dull red glow will be observed. If it is further heated up to about 5000 degrees-near the temperature of the sun's surface- it will radiate through the whole visible spectrum and it is said to be white hot.
By studying plates in thermal equilibrium, it can be seen that a black body is a body that absorbs all power incident on it.
A good example for explaining a black body is a cavity containing a small hole. All light falling on the hole passes into the cavity and is never reflected out, as it would have to endure a numerous number of reflections off the cavity walls. If the walls are absorptive (painted black), the cavity is turned to a perfect black body.