What is G₀ (quiescent phase) of cell cycle?

In adult animals some cells do not exhibit division and many other cells divide occasionally as to replace cells, which have been lost due to injury. These cells that do not divide further exit G₁ phase to enter an inactive stage called quiescent stage (G₀) of the cell cycle. Depending on the requirement of the organism the cells proliferate other wise the cells remain metabolically active but they do not proliferate.