ADP stands for Adenosine diphosphate, which is an important molecule in biology that plays a crucial role in the flow of energy in living cells. ADP is formed when ATP (Adenosine tri-phosphate), the main energy molecule cells use to power work, splits off one of its three phosphates. The energy holding that phosphate molecule is then released and available to do work for the cell. When the cell has extra energy, it stores that energy by reattaching a free phosphate molecule to ADP, turning it back into ATP. Therefore, ADP is an intermediate molecule in the ATP-ADP cycle, which is the process of converting ATP to ADP and back to ATP again. In summary, ADP is a molecule that is involved in the transfer of energy within cells.