1. 下降;减少;衰退;衰落
    If something declines, it becomes less in quantity, importance, or strength.

    e.g. The number of staff has declined from 217,000 to 114,000...
           员工人数从 21.7万减少到了11.4万。
    e.g. Hourly output by workers declined 1.3% in the first quarter...
           第一季度工人每小时的产量下降了 1.3%。

2. 婉拒;谢绝
    If you decline something or decline to do something, you politely refuse to accept it or do it.

    e.g. He declined their invitation...
           他谢绝了他们的邀请。
    e.g. The band declined to comment on the story...
           乐队拒绝对这一报道作任何评论。

3. 下降;减少;衰退;衰落
    If there is a decline in something, it becomes less in quantity, importance, or quality.

    e.g. There wasn't such a big decline in enrollments after all...
           注册人数终究还是没有出现如此大幅度的下降。
    e.g. ...Rome's decline in the fifth century...
           公元5世纪时罗马的衰落

4. 逐渐势微;正在衰退
    If something is in decline or on the decline, it is gradually decreasing in importance, quality, or power.

decline的翻译

    e.g. Thankfully the smoking of cigarettes is on the decline...
           令人欣慰的是，吸烟量在逐渐下降。
    e.g. He is still one of the world's most popular golfers, but his game is in decline.
           他仍是世界上最受欢迎的高尔夫球选手之一，但他的球技日不如前。

5. 走下坡路;开始衰退
    If something goes or falls into decline, it begins to gradually decrease in importance, quality, or power.

    e.g. Libraries are an investment for the future and they should not be allowed to fall into decline.
           图书馆是对未来的一项投资，不应任其日趋萎缩。