柯林斯词典定义如下：

1.N-COUNT （非婚的）爱人，情人

Someone's lover is someone who they are having a sexual relationship with but are not married to.

Every Thursday she would meet her lover Leon.

每周四她都会和情人利昂幽会。

2.N-COUNT 热爱者；爱好者；发烧友

If you are a lover of something such as animals or the arts, you enjoy them very much and take great pleasure in them.

She is a great lover of horses and horse racing.

她热爱马匹和赛马。

由上可见，lover通常表示与你有着性关系，却没有婚姻关系的人！如果你结婚了，别人就说你是某某的husband或者wife。相对来说，比较正式的单词是spouse，也相当于中文的“爱人”，偶尔会被用到。如果双方没有结婚，sweetheart也是比较恰当的表达爱意的用语。