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.
The downward curve on this graph shows the decline in sales.
Compound of 下降 (kakō, 'descent, fall') and 線 (sen, 'line').