The onset of densification of cellular solids represents the start of the cell wall interactions, which enhance the compressive resistance of a cellular solid. Currently, there exists ambiguity in the definition and uncertainty in the determination of the compressive strain, from which the densification regime starts. The onset strain of densification and the densification strain are defined and distinguished in the present study. Several commonly used methods to determine the onset strain of densification are examined. It is shown that the method based on the energy absorption efficiency curve gives unique and consistent results. Two types of energy absorption efficiency curves are identified. Further justifications of the use of the energy absorption efficiency method are provided for various types of cellular solids.