How to translate "曖昧" into English?
曖昧 = ambiguous romantic relationship

This phrase is in reference to a romantic relationship. Its usually from outsiders commenting on the relationship between a guy and a girl who are friends but may be more than that. 

You can kind of thinks about it as the sort of weird stage before you've had the DTR (determine-the-relationship) conversation. If you don't have that convo, then you may be perpetually 曖昧.