adultery

What is adultery ? Is it an act where one partner in a relationship has physical relationship with a partner outside the relationship? Yes it is. Does adultery cover the subject of mental/emotional adultery? Which one is worse?? Which one is more harmful?

Some of my friends love their partners, but are not able to keep it in their pants... I have a "friend". Now this girl is in love with her bf of some time, yet go figure she slept with another men, for no reason other then "why not". She loves her bf yet she still did. Go figure

On the other hand there is another friend, a distant distant relative who is in love with another woman yet is unable to have any intimate relationship with her because he cant bring him self to cheat on his wife. So is he not an adulterer already?