The One That Got Away.

1. The mate from a past relationship or friendship who, in the present reality, seems the ideal match, if it weren't for some force beyond your control, fate or otherwise, keeping you apart.

2. In virtually any context, someone you meet and share a significant encounter with who holds qualities akin to "the one" but for circumstance sake you are separated from; always after the fact.

3. More broadly, any love regret.

Credit: urbandictionary