(n) the extreme desire for something you can't have
(v) intensely desiring something you can't have

Usually used in reference to the girl/boy you have a crush on, yet will most likely never be able to have a relationship with them because they either do not feel the same way about you, or are already in a happy relationship.

Invented by two guys in Florida who needed a word that meant this, but found no such word existed and decided that would not do.