an invisible one sided line used as an excuse and crutch by women to politely turn down someone for whom they don't have romantic feelings for. Friends cross this "line" by making known their romantic feelings to a friend with the friend not feeling the same way. It's been said that once this line is crossed it changes the friendship perhaps forever, effectively ending the friendship. It's one-sided because men have no such "line of friendship" and will often welcome a female friend respectfully making known her romantic interest. It's an excuse or crutch because the "line of friendship" doesn't exist if the girl is interested also. Women are usually uncomfortable once this "line" is crossed. Men frankly don't care and can let the relationship continue as normal but that seldom happens.
Yo answer your phone! Carl is calling again.
I can't answer that. He crossed the line of friendship last week. I don't want to give him false hope.
Girl, you know if that was James calling there wouldn't even be a line!