Ellingham diagram: 

The graphical representation showing the variation of Gibbs energy with increase of temperature for the formation of oxide (oxidation) is known as Ellingham diagram.

OR 

The Ellingham diagram is the plot of free energy change ΔG^o against temperature for the reaction of a metal and other elements with one mole of gaseous oxygen at 1 atmosphere. 

Features of Ellingham diagram: 

The Ellingham diagram for oxides shows the following important features: 

a. The graph for the formation of a metal oxide is a straight line with an upward slope. 

b. There is sudden change in the slopes for some metal oxides, like MgO, ZnO and HgO. 

c. For a few metal oxides of mercury and silver, the graph is at the upper part in Ellingham diagram (Ag₂O and HgO). 

d. The graph for the formation of CO is straight line with the negative slope. This line intersects the lines of many metal oxides. 

e. The graph for the formation of CO₂ is straight line almost parallel to the temperature axis.