1 sink (down), drop (down), go down, recede, descend, decline; lower, settle:
The waters of the swollen river subsided. The volcano's centre subsided, leaving a caldera.
2 abate, quiet(en) (down), calm (down), moderate, let up, decrease, diminish, lessen, die (down or off or out), pass (away), wear off:
His enthusiasm for sky-diving subsided a bit after he broke both legs. When the clamour had subsided, the president rose to speak.