A term of affection referring to the other partner in a relationship. Often said to the male in reference to his girlfriend, implying that he is lesser. But often not.
language, it is used conversationally to demonstrate the other person in the relationship without referring to them as "him" or "her". This is sometimes useful if the other person is sensitive about being reduced to simple descriptors "Who's 'she'? The cat's mother
?" My better half
is also used in this context