To subside is to die down or become less violent, like rough ocean waves after a storm has passed (or your seasickness, if you happened to be sailing on that ocean).
Subside comes from the Latin prefix sub- (meaning "down") and the Latin verb sidere (meaning "to settle"). Subside is often used when a negative situation has improved significantly. For example, violence, disease, and unemployment can all subside. Here's hoping that they do.