a feeling of pleasure and also slight sadness when you think about things that happened in the past

Some people feel nostalgia for their schooldays. 有些人对自己的学生时代充满了怀旧之情。

Hearing that song again filled him with nostalgia. 再次听到那首曲子让他心中充满对往事的怀念。

a wave (= sudden strong feeling) of nostalgia 一阵强烈的怀旧感