noun
downward curve; declining trend
Often used figuratively for a decline in performance, popularity, or fortunes, as well as literally for a downward-sloping line on a graph.
彼の人気は下降線をたどっている。
His popularity is on a downward trend.
このグラフの下降線は売上の減少を示している。
The downward curve on this graph shows the decline in sales.