Here's the definition from Wiktionary

来源: 于 2012-02-16 08:15:04 [博客] [旧帖] [给我悄悄话] 本文已被阅读：次

[Note: the following is from wiktionary, not my imagination.]

Noun

sweetheart (plural sweethearts)

  1. A person who is always very kind.
    She is such a sweetheart, she never complains about my being late.
  2. A person very much liked or loved by someone, especially when both partners are young.
    John married his highschool sweetheart in 1981.