It is a supernatural spiritual union, 'One Flesh' ordained by God, witnessed by heaven, and in the earth, created by solemn vows taken by a couple here in the earth. One of my favorite books on this is: http://www.amazon.com/One-Flesh-Passion--Romance-Marriage/dp/0884193802/ref=sr_1_1?ie=UTF8&qid=1464268378&sr=8-1&keywords=one+flesh+by+bob+yandian