How would you define "emotional infidelity"?

There are many different takes on this issue. Some people believe that it isn’t possible to be just friends with someone of the opposite sex. They say that if you share parts of yourself with another person who is not your spouse and especially if it is secret, but even if it is not, that that is emotional infidelity. They feel it damages your relationship with your S/O. I think there is definitely a generational divide on this issue. But even young people who are fairly conservative in their view of marriage seem to hold to this view.
Then there are others who feel that by having a more open relationship, being trusting and tackling the jealousy issues involved will actually strengthen and improve your relationship.
People say that one person cannot be your everything. By having outside relationships, the wisdom goes, you enrich your relationship with your S/O because you have outside experiences to bring bring into it and to share.
What is your take on this issue? What should be acceptable? What is outside the bounds? Have you ever broken off a friendship with someone of the opposite sex because of boundary issues? If so, how did it work out?