G0, or "G-zero," refers to a phase in the cell cycle where cells are in a quiescent state. During this phase, cells are not actively dividing but are metabolically active. They can remain in G0 for extended periods, responding to environmental signals that may trigger them to re-enter the cell cycle and begin dividing again. Cells typically enter G0 from the G1 phase of the cell cycle, which is the first gap phase after cell division. This state is crucial for maintaining tissue homeostasis and allowing cells to conserve energy when conditions are not favorable for growth and division.