The Advance-Decline (A/D) Line is a market breadth indicator calculated by cumulatively adding the daily difference between the number of advancing stocks and declining stocks in an index or market. A rising A/D Line confirms a broad-based rally, while a falling A/D Line during a market uptrend (negative divergence) can signal underlying weakness and foreshadow a reversal. Technical analysts use the A/D Line alongside price indices like Nifty 50 to validate the sustainability and health of prevailing market trends.